Twelve adult female patients and 16 children who had undergone augmentation cystoplasty at least 2 years previously were studied to assess calcium balance and skeletal mineralisation. The serum and 24-h urinary calcium levels were measured and arterial blood gas analysis was performed in all patients. In children, skeletal mineralisation was assessed by serial study of their growth charts, comparing their growth centiles before and after cystoplasty. In adults, skeletal mineralisation was assessed by dual photon absorptiometry (DPA). As previously reported, all patients had a metabolic acidosis, usually with respiratory compensation. Serum and 24-h urinary calcium levels were all within the normal range. Growth charts of the 6 children with colocystoplasties showed an average of 20% reduction in growth potential in 3 of them. Growth charts in the 10 children with ileocystoplasties did not show any change in growth pattern. DPA bone scans in adults were all normal. These results suggest that if calcium is mobilised from bone in patients with a cystoplasty as a result of the metabolic acidosis, then it is reabsorbed from the bladder by the gut segment after an ileocystoplasty; colonic segments are less efficient than ileal segments, however, so that after a colocystoplasty skeletal demineralisation or a reduction in growth potential is more likely.