Vitamin D and mineral metabolism were studied in 21 adolescents and young adults with cystic fibrosis and the results were compared to those in 21 matched controls. All CF patients had been maintained on standard multivitamin supplements in combination with pancreatic enzyme replacement. Despite this supplementation, relative to control subjects the CF patients had a 36% reduction in serum 25-hydroxyvitamin D concentration, a slight but significant reduction in serum calcium concentration, evidence of calcium malabsorption with secondary hyperparathyroidism, and a 14% decrease in bone mass measured by the photon absorption technique. Currently accepted modes of pancreatic enzyme replacement and vitamin D supplementation are often inadequate to maintain normal mineral homeostasis in CF patients; additional measurements may be required to reduce the risk of clinically significant osteopenia concomitant with prolonged survival in CF.