It is reported on the bone marrow involvement of 389 malignant tumors investigated by sternal marrow aspiration. The frequency of skeletal metastases as revealed by routinely taken bone marrow aspiration was 20.3%. Bone marrow metastases were detected mostly in tumors of prostatic, pancreatic, renal and ovarian origin, in cancers of the breast and in melanoblastoma. Positive cytological findings increased to 29.0% in those metastatic tumors, where the frequency of metastases had been identified with clinical and different laboratory methods. In cases, where these methods detected the primary tumor only, the metastatic frequency was 13.4% in form of micrometastases, invisible by x-ray examination. The cell type of the metastatic tumor did not allow to localize the primary site of the disease. Non-specific reactions as eosinophilia and microgranuloma, both increased, are not specific but suspicious of skeletal metastases.