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/cm2, while that of untreated subjects was 56.69 g/cm2 (P < 0.02). In the SLE group, mean bone mass of CS-treated subjects (76%) was 61.47 g/cm2 and that of untreated subjects 62.36 g/cm2 (P > 0.1). Although patients with SLE required larger cumulative doses of 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.