OBJECTIVE Idiopathic hypercalciuria (IH) is associated with a decreased bone mineral density (BMD) both in children and adults. It is being increasingly recognized that IH may be a contributing factor to osteopenia and/or osteoporosis in adults. We studied BMD in girls with IH and in their mothers, also affected with IH, in order to evaluate the influence of genetic background on bone mass in the setting of IH. METHODS BMD was evaluated in 40 girls with IH and in their premenopausal mothers from whom they had inherited this disease. Urinary creatinine and calcium were measured by standard laboratory methods. BMD was determined by dual X-ray absorptiometry scanning of the lumbar spine (LS) and the femoral neck (FN), and values are expressed as Z- and T-scores. RESULTS A Z-score of <-1 at the LS was found in 42.5% of the girls, whilst in the mothers, a Z score of <-1 at the LS and/or FN was observed in 47.5% and a T-score of <-1 at the LS and/or FN in 62.5%. The Z-score at the LS was significantly lower in girls and their mothers compared to controls, although this finding did not apply for the Z-score at the FN in the mothers. Z-scores in the girls of mothers with osteopenia were significantly lower or there was a trend for the score to be lower than in the girls of mothers with normal BMD. There was a significant relationship between the Z-score of the girls and the T-score at the LS in the mothers (r = 0.32, p < 0.05). CONCLUSIONS We have observed a high prevalence of osteopenia in our population affected by IH, both in girls and in their mothers. We suggest that BMD should be measured during the third or fourth decades of life in those individuals with nephrolithiasis or with children diagnosed as having IH.