Using a structured interview, the mothers of patients with cycloid psychosis, manic depression and controls (40 mothers in each case) were investigated in order to assess the occurrence of maternal gestational infection and other obstetric complications during pregnancy with the affected child. The cycloid psychoses with low heritability and a good long-term prognosis were found to be significantly associated with first-trimester respiratory infection (i.e. influenza and febrile cold). Furthermore, maternal infection seems to predict an early onset in cycloids. In manic depression, we failed to identify a significant link with maternal gestational infection or other obstetric complications. These findings are discussed in the light of our previous reports of an excess of maternal gestational infections during the second trimester in chronic schizophrenics. Our results suggest that the exogenously induced disturbances of fetal brain maturation during the first trimester of gestation caused by maternal respiratory infection via live virus or disturbed maternal immune response are involved in the aetiology of cycloid psychoses.