We studied seven cases of malignant neoplasms, taken from various sites in the body, that were associated with marked granulocytosis. The seven cases were characterized clinically by marked granulocytosis with mature neutrophils, nonspecific inflammatory signs, and a rapid and progressive fatal course of the disease. The elevation of granulocyte count generally paralleled the increase in tumor size. Postmortem examination revealed no evidence of extensive bone marrow metastases or significant suppuration in any case. The bone marrows showed varying degrees of granulocytic hyperplasia with or without a shift to the left in the maturation series. Erythroid cell hyperplasia was observed in some cases, and in one instance there was an increase in immature eosinophils. The spleen showed various degrees of infiltration by neutrophils, from minimal to extremely marked; some spleens had foci of extramedullary hemopoiesis. Colony-stimulating activity was demonstrated in tumor extracts from three of these cases and from the serum in another case. Thus, it is suggested that marked granulocytosis in these patients was caused, at least in part, by colony-stimulating factor produced by the neoplastic cells.