BACKGROUND--Cystic fibrosis is a multisystem disease characterised by chronic pulmonary sepsis and malnutrition. To ascertain whether osteoporosis is a feature of cystic fibrosis in adult patients, total body and regional bone mineral density (BMD) was measured in a group of eight men and eight women aged 17-42 years. METHODS--Total body and regional BMD (lumbar spine L2-L4, femoral neck, trochanteric, and Ward's triangle), as well as total body fat and lean mass, were measured by dual energy x ray absorptiometry. A range of biochemical, lifestyle, and anthropometric variables was also assessed. RESULTS--Patients with cystic fibrosis had significantly reduced bone density at all sites compared with normal young adults. The mean reductions ranged from 7% at Ward's triangle to 13% at the trochanter. Body mass index (BMI) was positively correlated with BMD at four sites and disease severity negatively correlated with BMD at two sites. Other biochemical and anthropometric variables were not predictive of bone density. Total body fat mass was reduced by 30% compared with normal young adults. CONCLUSIONS--Bone density is decreased in adult patients with cystic fibrosis and BMI and disease severity are independent predictors of bone density.