In 31 children and adolescents with cystic fibrosis (CF), whose bone mineral density (BMD) was determined at the level of the lumbar spine, serum bone γ-carboxyglutamine acid (GLA) protein (BGP) was measured. A standard radioimmunoassay method gave for the studied CF population a median serum BGP concentration of 9 µg/l (range: 0.8-18.6) which corresponds to a median Z-score for age of-1.72 (range: -3.3 to +0.04). Thirteen patients presented a BGP concentration lower than 2 standard deviations (SD) under the mean for age. These patients did not differ from the others by their circulating thyroxine, triiodothyronine, 25-hydroxyvitamin D or parathyroid hormone levels. The lowering of the BGP concentration was neither related to the growth velocity nor to the severity of the lung disease, as assessed by lung scintigraphy. Six patients with a decreased BMD ( < 2 SD under mean for age) were found. Five of them had also a decreased BGP concentration. In conclusion, decreased serum BGP concentration was found in 42% of the studied CF patients and could not be related to hormonal abnormalities. Serum BGP had a low specificity (72%) in predicting osteopenia in young CF patients.