Levels of total lipids, cholesterol and triglycerides were followed in 30 children with malignant hematological diseases, or solid tumors; in addition, chylomicrons, VLDL, LDL and HDL were separated by electrophoresis on polyacrylamide gel. Nine of the 13 children with acute lymphoblastic leukaemia presented an increase of triglycerides and VLDL at time of diagnosis; all other parameters (cholesterol, HDL, LDL, chylomicrons) were normal. The lipid profile went back quickly to normal when hematological remission was obtained and remained stable throughout the course of the disease whatever the evolution might be. Five more patients in relapse were studied and did not present abnormalities. Similar results were obtained in 1/2 patients with acute myeloblastic leukaemia and in 3/5 patients with lymphoma. No lipid profile abnormality was observed in four patients with solid tumor and in one patient with histiocytosis X. The values of triglycerides and VLDL in pathological cases will be given. The endogenous nature of this disturbance is discussed as well as its relation with the rate of malignant lymphoid proliferation.