Tumour growth is angiogenesis-dependent. We found a high correlation between the extent of bone marrow angiogenesis, evaluated as microvessel area, and the proliferating (S-phase) fraction of marrow plasma cells, evaluated as labelling index (LI), in patients with multiple myeloma (MM) and in those with monoclonal gammopathies of undetermined significance (MGUS). Angiogenesis itself was significantly associated with active as opposed to non-active MM and MGUS. The highest microvessel area accompanied rapidly progressive MM with the highest LI. When a cut-off value of 2% or greater of the microvessel area was used, most patients with active MM were classified correctly. The risk of active disease in patients with MM increased in parallel with the microvessel area. A causal relationship between plasma cell growth, activity phase in MM and marrow angiogenesis is suggested. Since angiogenesis proceeds in step with the enlargement of plasma cell tumours and the activity phase in MM, its measurement could be a useful prognostic marker in patients with plasma cell proliferative disorders.