The observations of familial juvenile osteoporosis, presumably of genetic origin are exceptional. The authors report the observation of a 16-year old adolescent suffering from osteoporosis, confirmed by histomorphometry and decrease in bone density (lumbar vertebrae 0.79 g/cm2 and femoral neck 0.88 g/cm2: LUNAR DPX). We prescribed fluorine and calcium therapy. Lumbar bone density increases by 11% and bone density of the thighbone neck by 7.6%. We cannot rule out growth as a factor in the changes observed, given that the propositus is only 16. A densitometric investigation performed in 4 of his 12 brothers shows a decrease in the lumbar bone mineral content (from 61 to 94% expressed as Z score). A genotypic origin seems to be conceivable, especially since no other cause could be considered (endocrinal, alimentary...). On the other hand, there is no argument in favour of osteogenesis imperfecta disease. The bone densitometry is a useful diagnostic means to detect familial forms of osteoporosis.