Cell volume was measured using a Coulter counter in 42 cases of adult acute leukaemia, before any therapy. In every patient, the distribution was log-normal. A breakdown of the results with regard to the cell-type gives the following informations: The mean modal volume (M.M.V.) differs significantly from one type of leukaemia to another: the cell size in the myeloblastic type is larger than in the lymphoblastic, and smaller than in the monoblastic and myelomonocytic types. The dispersion of cell volumes, for a single patient, is low in acute lymphoblastic leukaemia, wider in myeloblastic cases and is even larger in myelo-monocytic patients. In 9 of the 10 acute myelo-monocytic leukaemias studied, the existence of two populations was shown by a double peak. Once relapse occurs, the cell volumes seem greater than during the first phase of the disease, at least in acute lymphoblastic leukaemia. No correlation has been found between the mean modal volumes and the labeling index, after incorporation of 3H-TdR.