Attention Deficit Hyperactivity Disorder: Circannual Rhythm in the Month of Birth. We studied the existence of circanual rhythm in the month of birth in participants with attention deficit hyperactivity disorder (ADHD) and analyzed the association between months of birth and ADHD. We analyzed 10,978 participants (1,778 ADHD/9,200 non-ADHD), aged 3 to 18 years. A consecutive sample of first visits to mental health between 1992 and 2021 is used. Participants with ADHD exhibit a significant circannual rhythm in the months of birth, with an acrophase in October. Being born in the last quarter of the year significantly increases the probability of being diagnosed with ADHD, controlling for the effect of sex and age. As the months of the year go by, there is a significant increasing linear trend of being diagnosed with ADHD, which is not observed neither in general population nor in cases without ADHD. Caution is necessary in diagnosis and treatment of children born in the last months of year, who, due to their greater immaturity compared to the other children in class, could be misdiagnosed and treated as ADHD.