Vitamin D and parathyroid hormone concentrations were assessed in 31 adults with cystic fibrosis (mean age 24, range 17-52 years), in 28 of whom the bone mineral index in the forearm was also determined. Serum 25-hydroxyvitamin D was subnormal in eight patients, of whom five were receiving vitamin D supplements in standard doses. 1,25-dihydroxyvitamin D and parathyroid hormone concentrations showed no consistent abnormalities. The bone mineral index was lower in patients with cystic fibrosis (p less than 0.02) than in controls. Five patients with unequivocally reduced bone mineral index had a subnormal mean serum 25-hydroxyvitamin D and significantly worse lung function than the other patients. There was a positive correlation between age and bone mineral index (r = 0.68, p less than 0.001). Thus a significant proportion of patients with cystic fibrosis living in a temperate climate are at risk of vitamin D deficiency. Osteopenia is common and is probably related to a combination of hypovitaminosis D, delay in puberty, hypo-oestrogenism in women, and reduced physical activity, rather than to secondary hyperparathyroidism. Since most patients with deficiency of 25-hydroxyvitamin D were receiving oral supplements, parenteral vitamin D supplementation may be appropriate for selected patients who are unable to maintain adequate 25-hydroxyvitamin D concentrations despite oral vitamin D supplements.