Background: Increased incidence and prevalence of autism spectrum disorder (ASD) over the last two decades have prompted considerable efforts to investigate its etiological factors. We examined an association between month of birth and ASD incidence. Methods: In a retrospective cohort of male children born from January 1999 to December 2008 in a large health organization in Israel (Maccabi Healthcare Services), ASD was followed from birth through December 2015. Results: Of 108,548 boys, 975 cases of ASD were identified. The highest rates (10.3 and 10.2 per 1000 male live births) were recorded for children born in May and August, and the lowest rates for February (7.6 per 1000 male live births). Among lower socioeconomic status households, boys born in August were more likely (OR=1.71; 95% confidence interval: 1.06 – 2.74) of being diagnosed with ASD than children born in January. Significantly higher rates were not observed for other months. Conclusions: In line with several previous studies, we found modestly higher likelihood of autism occurrence among male children of lower socioeconomic levels born in August.