This investigation was carried out on 100 bone marrow biopsies with metastases and 56 autopsies on patients with evidence of cancer. Leukoerythroblastosis was found in 44% of the patients with bone marrow mestastases and was more frequent in prostatic and gastric carcinoma. Moreover, the postmortem study of patients who died with cancer showed that leukoerythroblastosis was always the sign of bone marrow metastasis. A significant correlation was found between these blood changes and bone marrow fibrosis around the metastasis. Furthermore, leukoerythroblastosis seems caused by hepatosplenic extra medullary hematopoiesis.