This study was designed to evaluate the effects of low-dose corticosteroid (CS) therapy for rheumatoid arthritis (RA) and of high-dose CS therapy for systemic lupus erythematosus (SLE) on metacarpal bone mass in young (premenopausal) subjects. Ninety-eight patients with RA, 63 patients with SLE and 85 healthy controls of comparable age, race, sex and nutritional status were studied. Metacarpal bone mass was measured by radiogrammetry using a digitiser. In the RA patients, mean bone mass of CS-treated subjects (27%) was 52,31 g/cm(2), while that of untreated subjects was 56,69 g/cm(2) (P < 0,02). In the SLE group, mean bone mass of CS-treated subjects (76%) was 61,47 g/cm(2) and that of untreated subjects 62,36 g/cm(2) (P > 0,1). Although patients with SLE required larger cumulative doses bf CS for longer periods, their bone mass was higher than that of the RA subjects (P < 0,01). None of the patients had femoral neck or vertebral crush fractures. In RA, bone loss was probably a feature of severe disease rather than of CS therapy.