To compare the penetrability of methylprednisolone into lung tissue with that of prednisone, blood and bronchoalveolar lavage (BAL) fluid levels of methylprednisolone and prednisone were measured in 17 patients with a variety of lung diseases. To correct for variations in the quantity of lung fluid obtained by BAL, steroid levels were expressed in relation to creatinine concentrations. The level of methylprednisolone penetration in the pulmonary parenchyma, expressed by the slope of the relation between blood and BAL fluid, was 0.5 (r = 0.8; p less than 0.03). By contrast, despite serum levels of between 59 and 219 ng/ml, prednisone could not be detected in the BAL fluid in 3 patients; the overall correlation between blood and BAL fluid for all patients was r = 0.5 (slope = 0.3; p less than 0.1). Thus methylprednisolone is better able than prednisone to penetrate lung acini.