This population-based study presents socioeconomic differences in psychiatric inpatient care by diagnosis. Inpatient care among the Finnish population aged 25-64 years was studied using data from the Finnish National Hospital Discharge Register. All major mental disorders in the ICD-9 were included in the study. The socioeconomic status of individual patients was defined by years of education in the population census. Discharge rates, first-time admission rates and hospitalization risk were usually 2- to 4-fold higher in the low educational group compared with the highly educated population. The socioeconomic gradient was steepest for schizophrenia. No gradient was observed for major affective disorders. However, bipolar disorder was most common in the highest educational category. For most conditions, the socioeconomic gradient among women was lower than among men. In Finland hospitalization was more common among low than high socioeconomic groups for most mental disorders and most indicators of inpatient care. Most of these differences are fairly consistent with previous data on socioeconomic gradients in the prevalence of mental disorders.