Bone mineral content, bone width, and their ratio were measured in patients with cystic fibrosis (CF) using monoenergetic photon absorptiometry. Serial measurements of the radius and ulna were made in 27 patients with CF and were compared with 968 age-matched controls. Demineralization was found in 37% of the boys and 63% of the girls. Patients under age 10 years had normal bone mineral content and nine of 15 patients aged 13 or older were demineralized (P less than .01). Demineralization correlated with the extent of weight reduction in patients (P less than .001). Patients most likely to be demineralized were adolescent girls. To our knowledge, this is the first report of bone mineral status of children with CF, and the results indicate that a sizable proportion of these patients may be demineralized without overt rickets.