Between January 1987 and October 1989, 561 consecutive untreated patients with monoclonal gammopathy of undetermined clinical importance (MGUS) (n = 295) or with multiple myeloma (n = 266) were evaluated in a multicentre trial. Both bone marrow biopsy and aspiration (performed at different anatomical sites) were required at presentation. Bone marrow biopsy data indicated that changes in bone marrow composition from MGUS to early multiple myeloma and to advanced multiple myeloma followed a precise pattern, including an increased percentage of bone marrow plasma cells (BMPC%), a shift from plasmocytic to plasmoblastic cytology, an increase in bone marrow cellularity and fibrosis, a change in bone marrow infiltration (becoming diffuse rather than interstitial), a decrease in residual haemopoiesis and an increase in osteoclasts. In multiple myeloma the BMPC% of biopsy specimens and aspirate were closely related, although in 5% of cases the difference between the two values was greater than 20%. Some histological features were remarkably associated with each other. For example, BMPC% was higher in cases with plasmoblastic cytology, heavy fibrosis, or reduced residual haemopoiesis. Anaemia was the clinical characteristic most influenced by bone marrow histology. The BMPC% was the only histological variable which affected the greatest number of clinical and laboratory characteristics, including, besides haemoglobin concentration, erythrocyte sedimentation rate, radiographic skeletal bone disease, and serum concentrations of monoclonal component, calcium, beta 2-microglobulin and thymidine kinase activity. These data indicate that comparative bone marrow histology in monoclonal gammopathies has clinical importance.