The change of bone mass during maintenance haemodialysis was studied in 20 patients. Iliac crest biopsies were obtained at a 9 - 16 months interval. Two biopsies were taken in order to give an independent estimate of the sampling error. 22 individuals in the same age group without skeletal disease who died from traffic accidents or from violent deaths were used as controls. Undecalcified sections of bone (Masson-Goldner-stain) were evaluated by micromorphometry. At the beginning of the study osteosclerosis (i.e. volumetric density of bone above x +/- 2 SD in control patients) was found in 14 out of 20 patients. Volumetric density of bone only increased in one of the patients during the observation period. A significant decrease of volumetric density of bone was found in 5 out of 20 patients. This could be attributed to prolonged immobilisation in two cases and to inadequate protein- and calorie-intake in two cases. No predisposing factor could be found in the fifth case. It is concluded that loss of spongy bone mass (osteopenia) is not an invariable result of haemodialysis.