In order to quantify bone changes which occur in multiple myeloma, undecalcified transiliac bone biopsies from 118 myelomatous patients were analysed by histomorphometric methods. Osteoclastic resorption surfaces were increased compared with controls, and the number of osteoclasts/mm2 of bone section was significantly greater in the areas massively invaded by plasma cells than in less invaded areas. The osteoid surfaces were also increased and the percentage of trabeculae that exhibited tetracycline labelling was also greater, indicating increased formation surfaces. However, reduced thickness of the osteoid seams and a low calcification rate, measured after tetracycline double labelling, suggests a reduced activity for each osteoblast. The mean trabecular bone volume was not reduced as compared with controls, but the biopsies showed a heterogeneous distribution of osteolytic and osteosclerotic areas. In the invaded areas, no major histomorphometric difference was found between patients receiving chemotherapy and untreated patients, demonstrating that if usual chemotherapies reduce the tumour mass, they do not improve histological bone lesions in areas still invaded by plasma cells.