Using microcalorimeters of the thermopile conduction type heat production was measured in lymphocytes from peripheral blood in 8 normals and 10 patients with chronic lymphocytic leukaemia (CLL). The heat production per CLL lymphocyte was lower (1.8 pW/cell) than that found in normal lymphocytes (2.6 pW/cell). Due to the high numbers of lymphocytes in the peripheral blood the estimated heat production of the intravascular lymphocyte pool in CLL was considerably higher than in normals. Since the circulating lymphocytes constitute a minute fraction of the total lymphoid mass in CLL it is suggested that the accumulation of metabolically active lymphocytes in blood and tissues may explain the common clinical signs of hypermetabolism in this disease. The results also indicate that calorimetry may be a useful technique for metabolic studies in suspensions of malignant cells.