Cystic fibrosis (CF) patients often have low bone mineral density (BMD) and may suffer from fractures and kyphosis. The pathogenesis of low BMD in CF is multifactorial. To study bone metabolism, we collected fasting serum and urine from 50 clinically stable CF adults (mean age 28 years) and 53 matched controls to measure markers of bone formation and bone resorption. The CF subjects had moderate lung disease (FEV1: 46.1 +/- 18.6% predicted) and malnutrition (BMI: 20.0 +/- 3.3 kg/m2). Only 3 subjects had normal BMD. CF subjects had higher urinary N-telopeptides of type I collagen (81.0 +/- 60.0 vs 49.0 +/- 24.2 nm BCE/mmol creatinine, p = 0.0006) and free deoxypyridinoline (7.3 +/- 5.0 vs 5.3 +/- 1.9 nM/mM, p = 0.004) levels than controls. Serum osteocalcin levels were similar in the two groups, a result confirmed by two immunoassays that recognize different epitopes on osteocalcin. Serum bone-specific alkaline phosphatase levels were elevated in CF patients (32.0 +/- 11.3 vs 21.8 +/- 7.0 U/l, p < 0.0001), but were much more closely associated with serum total alkaline phosphatase levels (r = 0.51, p = 0.001) than with age or gender. Parathyroid hormone levels were elevated (p = 0.007) and 25-hydroxyvitamin D levels were depressed (p = 0.0002) in the CF patients in comparison with controls. These results indicate that adults with CF have increased bone resorption with little change in bone formation. Medications that decrease bone resorption or improve calcium homeostasis may be effective therapies for CF bone disease.