A case of Burkitt's lymphoma with granulocytosis including immature cells and a high level of C-reactive protein (CRP) is reported. The activity of interleukin-1 alpha (IL-1 alpha) was detected in the sera, although its activity was not detected in the supernate of the cultured lymphoblasts. The lymphoblasts stimulated production of IL-1 alpha from human umbilical endothelial cells. Therefore, the lymphoblasts infiltrating bone marrow may have stimulated production of IL-1 alpha from bone marrow stroma cells, and IL-1 alpha may have induced granulocytosis and a high level of CRP.