Purpose: To examine the association between autistic spectrum disorder (ASD) and seasons of conception and birth in a UK birth cohort: Avon Longitudinal Study of Parents and Children (ALSPAC). Methods: Seasons of conception and birth were compared in children with and without ASD with season grouped as follows: spring (March–May); summer (June–August); autumn (September–November) and winter (December–February). Results: A total of 86 children with ASD were identified in the ALSPAC cohort giving a prevalence of ASD of 61.9 per 10,000. There was some evidence for an excess of children with ASD being conceived during the summer months with a rate per 1,000 conceptions of 9.5 in summer compared to 5.1, 4.6, 5.7 in spring, autumn and winter, respectively. A doubling of the odds was suggested for summer compared to autumn (Odds ratio 2.08 [1.18, 3.70]). In agreement with previous research, there was a corresponding peak in spring births. Conclusion: Conception during the summer months was associated with an over-representation of children with ASD in this UK birth cohort. There was also an association between ASD and spring births. Further investigation of seasonal influences on the aetiology of autism is required to identify possible factors in the environment, and their mechanisms and timings.