Bone marrow biopsies of 678 untreated patients with established malignant non-Hodgkin's lymphomas (ML) were investigated. The bone marrow was involved in 468 cases, an overall frequency of 69%. The Kiel classification of the ML (based on lymph node histology) was applied and the biopsies were classified: ML lymphocytic 36%, ML 'hairy cell' 24%, ML lymphoplasmacytic/cytoid 24%, ML centrocytic 6%, ML centroblastic/centrocytic 4%, ML lymphoblastic (without ALL) 3%, ML centroblastic 2% and ML immunoblastic 1%. The life tables of the patients were similar whether classified according to the histology of the lymph node or the bone marrow. A multivariate computer based analysis of both clinical and histological data was performed to test their prognostic relevance. The cell type, the proliferation pattern and the extent of infiltration in the bone marrow all proved to be factors of prognostic significance. The results indicate that classification of the ML based on lymph node histology is applicable to the bone marrow, is reproducible and has prognostic significance. Consequently, a bone marrow biopsy is a useful clinical tool for staging and for histological classification of patients with ML.