As adults with cystic fibrosis (CF) have enjoyed incremental increases in longevity over the last few decades, they have also been suffering from low bone density and its clinical manifestations, fractures and kyphosis. We conducted a placebo-controlled, randomized, double-blinded trial of alendronate (10 mg/day orally) (n = 24) compared with placebo (n = 24) for 1 year in 48 patients to improve bone mineral density at the spine as the primary endpoint. All patients received 800 IU of cholecalciferol and 1,000 mg of calcium carbonate. Both groups were similar in age, sex, CF mutations, bone density T scores, renal function, and body mass index at study onset. The alendronate-treated patients gained (mean +/- SD) 4.9 +/- 3.0% and 2.8 +/- 3.2% bone density after 1 year versus placebo, which lost (mean +/- SD) 1.8 +/- 4.0% and 0.7 +/- 4.7%, in spine and femur bone density, respectively (p < or = 0.001 for the spine; p = 0.003 for the femur). Urine N-telopeptide, a bone resorption marker, levels declined in the treatment group more than in the control group (p = 0.002), consistent with the known antiresorptive effects of bisphosphonates. Alendronate was more effective than placebo in improving spine and femur bone mineral density and is a promising agent for the long-term prevention and management of bone disease in patients with CF.