The metabolism of cortisol by white cells from 20 patients suffering from various forms of leukemia has been studied in vitro and compared with the changes produced by normal leukocytes. Known concentrations of cells were incubated with (4-14C) cortisol in tissue culture medium and the products were isolated by paper chromatography. The main metabolites produced by both normal and malignant cells were 20α- and 20β-dihydrocortisol and tetrahydrocortisol. Cells from acute lymphatic and myeloid leukemia were 4 times as active as the normal leukocytes in reduction of the C-20 carbonyl and twice as active in reduction of ring A. The cells from patients with chronic leukemia were intermediate in their degree of metabolic activity. In contrast, the transformation of 65% of normal lymphocytes to “blast” forms by phytohemagglutinin was accompanied by no significant increase in the metabolism of cortisol. The pattern of cortisol metabolism by both normal and malignant leukocytes differed from that seen in other extrahepatic tissues by the presence of ring A reduction and the absence of oxidation of the 11β-hydroxyl group.