Shamatha (single-pointed concentration) and vipassana (non-judgmental awareness and observation of all thoughts and feelings - much easier when you've first used shamatha to quiet down your mind enough to notice each new thing and focus your attention on attention itself) are the two meditation forms I practise.
(They don't use the words shamatha or vipassana but "mindfulness" is generally synonymous for vipassana, and shamatha precedes vipassana both traditionally and in the studies discussed below.)
This paper explains why there is ample theoretical reason to test the effects of mindfulness on ADHD given what is known about the brains and cognitive and affective profiles of ADHD patients and the effects of meditation on the brain, cognition and affect.
This lady is a psychiatrist who was involved in the study below and runs a programme of mindfulness training for ADHD.
Dr. Lidia Zylowska's website
This one, like most preliminary studies, has no control group, so really just serves as a suggestion that it would be worth looking at further with rigorous, more conclusive studies (which in this case would probably involve using both groups given non-meditative relaxation methods and groups given no method as controls).
Mindfulness Meditation Training in Adults and Adolescents With ADHD
A more indepth look at the findings of the above study by a clinical psychologist.
Mindfulness meditation for adults and teens with ADHD
"The current findings support that a large portion of variability in trait mindfulness can be explained by ADHD status and personality traits of self-directedness and self-transcendence. It further suggests that interventions that increase mindfulness might improve symptoms of ADHD and increase self-directedness and/or self-transcendence."
Mindfulness and attention deficit hyperactivity disorder
Small study of attention with non-ADHD group. Unlike the above study with people with ADHD, this study found that mindfulness meditation also improved working memory, not just sustained attention. The working memory deficit could be one aspect of ADHD that cannot be improved this way, or else the mindfulness programme used could have been in some way superior in the study of people without ADHD.
The Impact of Intensive Mindfulness Training on Attentional Control, Cognitive Style, and Affect
I recommend reading as much of the first paper as you can, but here are two paragraphs of interest:
The two types of meditation may be linked to different systems of attention. Concentrative meditation has been linked to the orienting and conflict monitoring system, proposed by Posner and Petersen as the dorsal attention system, which is described as a voluntary attention system activated by presentation of cues indicating perceptual and response features of stimuli to which participants should direct their attention. In contrast, mindfulness meditation can be linked with the alerting system, or the ventral attention system, which is described as an alerting system that is activated during abrupt changes in sensory stimuli and detection of salient targets, especially when they are unexpected, are outside of the focus of attention, and have low probability of occurrence.
That 'alerting system' is also problematic in ADHD:
Some of the sustained attention problems among ADHD individuals may also be linked with deficits in alerting mechanisms, which are critical for normal cognitive functioning. Earlier work using spatial-orienting tasks suggested that ADHD individuals show difficulty in maintaining the alert state (sustained attention) in the absence of a warning signal. More recent studies using the Attentional Network Task (ANT) have replicated problems with alerting in ADHD, again mostly due to the inability of the individual to maintain the alert state when no warning signal was used. Other studies using tasks similar to ANT have also shown some evidence of abnormalities in alerting and/or executive control in ADHD in terms of slowed response times to abrupt visual cues, especially when faced with conflicting spatial cues.
Anecdotally, I very much relate to what's described in that last paragraph. There's also evidence that people with ADHD (and the frequently comorbid borderline personality disorder - 60% of borderlines have a history of usually undiagnosed ADHD) have below average self-monitoring ability and subsequently inaccurate self-reporting (also suggested by that study of mindfulness levels found in ADHD adults linked to above). Teaching self-monitoring strategies improves attentiveness to task and productivity, according to a small study. Mindfulness is basically advanced self-monitoring, so it should be helpful too. This is what I've found. I'm getting better at noticing when my attention is moving or becoming split in the first place and noticing what triggered it (vipassana-related), and at peacefully acknowledging so, noting the task-irrelevance of the distraction, and returning focus with less fight-back from the distraction and discomfort (shamatha-related). The more quickly this all happens, the less I miss of what's going on around me in the process.