OBJECTIVE: No clear answer has yet been attained as to the influence of prenatal exposure to infection on autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD), either alone or as co-occurring issues. The current study examined links between hospital-recorded and maternal-reported prenatal infections and ASD, ADHD, and co-occurring ASD and ADHD. METHODS: Participants were n = 15,462 children and mother pairs from the Millennium Cohort Study (MCS), a population-representative UK sample. RESULTS: Findings show associations between maternal-reported infections and ASD, and some evidence of links with ADHD and co-occurring ASD and ADHD. Hospital-recorded infections were not found to be associated with ASD, ADHD, or their co-occurrence. Agreement between hospital-recorded and maternal-reported infections was low, which may explain the discrepant findings. CONCLUSION: Prenatal maternal infections may be associated with increased odds of ASD and ADHD. Findings point to the importance of drawing on multiple sources of information when ascertaining prenatal infection status.