This study of 96 bone biopsies carried out in 84 patients with various malignant solid tumours showed 27 normal bone marrows, 21 hypoplastic marrows, 34 irritant marrows and 14 metastatic marrows. All the patients in the last group and 75% of those with signs of irritation already had other metastases. Tolerance to chemotherapy was better in the normal marrows than in the other cases. There was a significant relationship between the irritant or metastatic or metastatic aspect and the existence of peripheral leucoerythroblastosis (metastatic marrows) or increased platelets (irritant appearances). A significant difference existed during the first five months of survival between these cases with a metastatic marrow and those with an iritant marrow. Bone biopsy should, in our opinion, form part of the routine investigation of cancer patients. It may identify metasine investigation of cancer patients. It may identify metastases where radiography and bone scans have failed.