To determine if osteoporosis is prevalent among patients with cystic fibrosis, we compared the vertebral bone density measured by quantitative computed tomography in 57 such patients (29 male, 28 female, aged 3 to 21 years) with those of an age-, race-, and sex-matched control group of 57 healthy subjects. Patients with cystic fibrosis had significantly lower bone density (10% lower, p less than 0.001) than in controls. The decrease in bone density in patients with cystic fibrosis was unrelated to age. Shwachman clinical evaluation scores (based on case history, pulmonary physical findings, growth, and x-ray findings) correlated positively with age-standardized bone density values (p less than 0.01). Male patients had substantially lower bone density than did female patients (p less than 0.02), but bone density differences related to gender were not significant when effects of disease severity were controlled for. Decreased bone density was more common in patients with poor nutritional status as determined by anthropometric measurements (p less than 0.05). We conclude that osteoporosis is a frequent complication in children with cystic fibrosis regardless of their age and is more prevalent in patients with greater disease severity.