Seasonal variations in the birth patterns of 140 ADHD boys compared to 120 controls were studied at the Massachusetts General Hospital, Boston, with particular reference to issues of psychiatric and cognitive comorbidity and familiality. Statistically significant peaks for September births were noted in ADHD children with comorbid learning disabilities and in those without psychiatric comorbidity. A trend toward an increase in winter births was also evident. A first-trimester viral hypothesis for ADHD is suggested. 
COMMENT. Exposure to viral infections during winter months in the first trimester of fetal life or at the time of birth may be a predisposing factor in 10% of ADHD subjects having comorbid learning disabilities. Prenatal or perinatal infection as a possible cause of ADHD requires further evaluation.
Familial transmission of ADHD is supported by a prospective four-year follow-up study of siblings of ADHD children, one fourth having developed ADHD and one half showing evidence of school failure.