Fasting urine calcium excretion was measured in 15 asthmatic patients receiving long-term glucocorticoid therapy (steroid group) and in age- and sex-matched asthmatics not receiving these drugs. In the steroid group, the mean urinary calcium/creatinine ratio and the mean calcium excretion per liter of glomerular filtrate (Ca<sub>E</sub>) were both approximately twice the control values (p < 0.005). When Ca<sub>E</sub> was plotted as a function of serum calcium it more often exceeded the mean normal value in the steroid-treated patients than in the controls (p < 0.05), suggesting a reduction in tubular calcium reabsorption. Calculation of the tubular maximum for calcium reabsorption confirmed a significant reduction in the glucocorticoid-treated patients (p < 0.005). It is concluded that glucocorticoid drugs probably inhibit the tubular reabsorption of calcium and that this is likely to contribute to the development of osteoporosis in patients receiving this treatment.