Udgivelsesdato: 2007-Mar ; BACKGROUND: The etiologies of autism spectrum disorder and many neurodevelopmental disorders are largely unknown. The detection of a seasonal variation of birth of children diagnosed with a certain disorder could suggest etiological factors that follow a seasonal pattern. We examined the seasonal variation of births of children diagnosed with any of 4 common childhood neuropsychiatric disorders: autism spectrum disorder, hyperkinetic disorder, Tourette syndrome, and obsessive-compulsive disorder. METHODS: The study cohort consisted of all children born in Denmark from 1990 through 1999 identified in the Danish Medical Birth Register (n = 669,995). Outcome data consisted of both inpatient and outpatient diagnoses reported to the Danish National Psychiatric Registry from 1995 through 2004 using the International Classification of Diseases, 10th edition, diagnostic coding system. Logistic regression combined with spline (a smoothing method) was used to estimate the variation with season of birth for each disorder. Estimates of risk of each disorder with season of birth were adjusted for differences in follow-up time and change in incidence over time. RESULTS: No convincing variations in season of birth were observed for any of the 4 disorders, or for the autism-spectrum-disorder subtypes. CONCLUSION: Although we cannot rule out the possibility of seasonal variation of birth for a range of childhood neurodevelopmental disorders, we find little evidence that seasonal environmental factors are related to these disorders.