We have investigated eight different vitamins, making this the most extensive study of vitamins in ADHD so far and also the first study of many of the vitamins in relation to ADHD. Lower levels of the vitamins B2, B6 and B9 were associated with the ADHD diagnosis, and lower levels of B2 and B6 were also associated with a higher symptom load. However, lower vitamin B6 and B9 levels were associated with smoking, which may have influenced the results. The lowest and highest values of vitamin D were overrepresented in ADHD patients, indicating a higher prevalence of vitamin D deficiency, possibly also treated. We have, however, no clinical information on supplement treatment, and group differences in the time of collection of blood samples may have influenced our results.