In this study serum cholesterol was measured in different types of human hematologic malignancies characterized by a wide range of cell proliferation. In all tumoral types a significant decrease of HDL cholesterol was observed, whereas total serum cholesterol generally remained unchanged. Another interesting observation of our study was the apparent inverse correlation between the extent of cell proliferation in these neoplastic disorders and the level of HDL cholesterol. Since a decrease of HDL cholesterol was previously observed, in our laboratory, in different experimental models of normal and neoplastic cell proliferation, we suggest that the decrease of HDL cholesterol may be a generalized phenomenon related to massive cellular growth in normal and malignant processes.