When blood cells are projected into an image plane their contours appear as borderlines of irregular shape with the property of the statistical self-similarity. The dimension D of these cell perimeters may be fractal rather than an integer as in the Euclidean space. The fractal dimension D for different q'order moments was evaluated by means of a probabilistic method after recording EM image profiles of normal peripheral blood mononuclear cells (PBMN) and mature lymphocytes and also of lymphoblasts isolated from humans with acute leukemia or obtained by lectin stimulation in vitro. PBMN cells, CD 4 and CD 8 T-lymphocytes showed a fractal dimension D quite similar among five moments, ranging from 1.23-1.17. Cells of hairy-cell leukemia with highly convoluted morphology displayed D comprised between 1.32-1.36, whereas blasts of T or B acute lymphoblastic leukemia were characterized by a smaller D of 1.11 and 1.13-1.16 respectively. When normal T-lymphocytes were transformed into blasts by lectin PHA stimulation in vitro, the fractal dimension D was significantly reduced and found close to the value recorded on pericellular contour of blasts from acute T-lymphoblastic leukemia.