Bone mineral density (BMD) in cystic fibrosis (CF) patients falls progressively below normal with advancing age, in part due to steroid administration, low levels of sex hormones, chronic inflammatory disease, physical inactivity, and chronic malabsorption of calcium and/or vitamin D. The purpose of this study was to compare the fractional absorption of (45)Ca and urinary excretion of calcium in CF subjects and normal controls following a high-calcium breakfast containing (45)Ca. Seven young men and 5 young women with CF with pancreatic insufficiency were studied on two separate occasions, with and without administration of pancreatic enzymes. Eleven healthy young adults with normal BMD measurements served as controls. Mean T-scores at the lumbar spine and femur were significantly lower in the CF subjects (p<0.002). Following baseline, fasting collections, timed serum and urine samples were obtained for 5 h after the meal. Fractional absorption (FA) of (45)Ca was estimated by the method of Marshall and Nordin. At baseline, CF subjects had lower mean serum 25-hydroxyvitamin D, calcium and albumin values (p<0.03 for each), slightly, but not significantly (p = 0.12), lower albumin-corrected calcium values, equivalent serum 1, 25-dihydroxyvitamin D values and a trend toward a higher mean serum parathyroid hormone (PTH) value (p = 0.10). Without pancreatic enzymes, CF subjects showed significantly impaired calcium absorption (5 h FA: 11.8 +/- 0.5 for controls vs 8.9 +/- 0.2 for CF subjects, p = 0.02) and excretion (4 h excretion: 0.20 +/- 0.08 mg Ca/mg creatinine for controls vs 0.16 +/- 0.09 mg Ca/mg for CF subjects, p = 0.025). Addition of pancreatic enzymes did not fully compensate for this deficiency. In addition, CF patients had higher serum PTH values after a high-calcium meal (p = 0.03), suggesting mild secondary hyperparathyroidism. Altered calcium homeostasis is likely to be a factor in the development of bone disease in CF patients.