Hormonal imbalances in utero may render males more vulnerable to neurodevelopmental disorder (ND) than females. Since hormonal activity can be influenced by photoperiod, the relationship between season of conception and incidence of ND in offspring was examined within 11,578 mother/child pairs. Fall conception significantly elevated the odds for mental retardation, reading, arithmetic disability, or performance aptitude deficits (but not seizures, articulation disorder, cerebral palsy, or verbal aptitude deficits), and decreased the odds for reading talent (even when socioeconomic class, prenatal visits, infections, fever, vomiting, edema, anemia, and weight loss were covaried). Since the seasonality effect was not stronger in males, and was not specific to those NDs caused by left hemisphere dysfunction, the predictions of Geschwind and Galaburda (1985a, 1985b, 1985c) were not confirmed.