A total of 146 patients with newly diagnosed acute lymphoblastic leukemia (ALL) occurring over a 9 1/2-yr period were reviewed; four of these patients (2.7%) had blast cells that stained positively with Sudan Black B. These four, two adults and two children, were all B type. Electron microscopy was performed on three of these four cases and showed the sudanophilia to be due to lipid droplets. The lipid droplets were also demonstrated in one of the three cases by Oil Red O stain. Therefore, Sudan Black B used alone to differentiate lymphoid from myeloid leukemia may be misleading.