We analyzed data from case groups of 538 children with autism spectrum disorders (ASD) and 163 with developmental delays (DD), and from 421 typically developing controls to assess associations with maternal influenza or fever during pregnancy. Exposure information was obtained by telephone interviews, and outcomes were clinically confirmed. Though neither ASD nor DD was associated with influenza, both were associated with maternal fever during pregnancy: OR's (odds ratios) were 2.12 (95% CI 1.17, 3.84) and 2.50 (95% CI 1.20, 5.20) respectively. However, the fever-associated ASD risk was attenuated among mothers who reported taking antipyretic medications (OR = 1.30, 95% CI 0.59, 2.84), but remained elevated for those who did not (OR = 2.55, 95% CI 1.30, 4.99).