We conducted a nested case-control study including 407 cases and 2,075 frequency matched controls to investigate the association between maternal infections during pregnancy and risk of autism spectrum disorders (ASD). Cases, controls, and maternal infections were ascertained from Kaiser Permanente Northern California clinical databases. No overall association between diagnoses of any maternal infection during pregnancy and ASD was observed [adjusted odds ratio (OR[subscript adj]) = 1.15, 95 % confidence interval (CI) 0.92-1.43]. However, women with infections diagnosed during a hospital admission (OR[subscript adj] = 1.48, 95 % CI 1.07-2.04), particularly bacterial infections (OR[subscript adj] = 1.58, 95 % CI 1.06-2.37), were at increased risk of delivering a child with ASD. Multiple infections during pregnancy were associated with ASD (OR[subscript adj] = 1.36, 95 % CI 1.05-1.78).