OBJECTIVE: Although major advances have been made in the diagnosis and treatment of mental disorders in primary care, few population-based investigations have focused on the obstetrical sector. This study examines the occurrence of chart-recorded psychiatric discharge diagnoses among all women delivering in California hospitals in 1992. METHOD: The authors undertook an archival analysis of the California Health Information for Policy Project data set, which consists of linked hospital discharge and birth certificate data for 580,282 deliveries. Frequencies of ICD-9 psychiatric diagnoses were ascertained. RESULTS: Among all women delivering, 1.5% received psychiatric or substance use diagnoses. Of diagnoses recorded, 75% were substance use disorders, 21% were classified generically as "mental disorder of pregnancy," and other psychiatric disorders accounted for 4%. CONCLUSIONS: The occurrence of psychiatric diagnoses in these women is markedly lower than expected, suggesting an underreporting of psychiatric disorders at delivery. Further investigations into the detection of mental disorders in the obstetrical sector are needed.