The authors examined whether serologically confirmed maternal exposure to influenza was associated with an increased risk of bipolar disorder in the offspring and with subtypes of bipolar disorder, with and without psychotic features. The study used a nested case-control design in the Child Health and Development Study birth cohort. In all, 85 individuals with bipolar disorder were identified following extensive ascertainment and diagnostic assessment and matched to 170 comparison subjects in the analysis. Serological documentation of maternal exposure to influenza was determined using the hemagglutination inhibition assay. No association was observed between serologically documented maternal exposure to influenza and bipolar disorder in offspring. However, maternal serological influenza exposure was related to a significant fivefold greater risk of bipolar disorder with psychotic features. The results suggest that maternal influenza exposure may increase the risk for offspring to develop bipolar disorder with psychotic features. Taken together with earlier associations between prenatal influenza exposure and schizophrenia, these results may suggest that prenatal influenza is a risk factor for psychosis rather than for a specific psychotic disorder diagnosis.