Objectives To assess the risk of autism spectrum disorders (ASD) in children born after assisted conception compared with children born after natural conception. Design Population-based follow-up study. Setting All children born alive in Denmark 1995-2003. Participants 588 967 children born in Denmark from January 1995 to December 2003. Assisted conception was defined as in vitro fertilisation (IVF) with or without intracytoplasmic sperm injection and ovulation induction (OI) with or without subsequent insemination. Children exposed to IVF or OI were identified in the IVF Register and in the Danish Drug Prescription Register. Main outcome measures A diagnosis of ASD in the Danish Psychiatric Central Register. Results 33 139 (5.6%) of all children born in Denmark in 1995-2003 resulted from assisted conception, 225 of whom (0.68%) had a diagnosis of ASD. Of the 555 828 children born in this period after natural conception, 3394 (0.61%) had a diagnosis of ASD. The follow-up time was 4-13 years (median 9 years). In crude analyses, children born after assisted conception had an increased risk of a diagnosis of ASD: crude hazard rate ratio (HRR) 1.25 (95% CI 1.09 to 1.43). In analyses adjusting for maternal age, educational level, parity, smoking, birth weight and multiplicity, the risk disappeared: adjusted HRR 1.13. (95% CI 0.97 to 1.31). However, subgroup analyses that suggest possible associations in women who received follicle stimulating hormone indicate the need for further study. Discussion This population-based follow-up study found no risk of ASD in children born after assisted conception.