p53 protein expression has been investigated by immunohistochemistry in 58 patients with leukemia. Seven of 24 cases with acute myeloid leukemia (AML), 3 of 15 cases with chronic lymphocytic leukemia (CLL), one of 11 cases with chronic myeloid leukemia (CML) and 4 of 8 cases,vith acute lymphoid leukemia (ALL) had p53 protein expression. Of patients having p53 expression, one case with AML had refractory anemia with excess blasts-transformation (RAEB/t), one case with CLL had Richter's syndrome and another one with CML was in accelerated phase. Finally, 26% of leukemia cases had p53 protein expression. It may be concluded that p53 protein abnormalities may have an important role in leukomogenesis and in the development of more malignant clones in chronic leukemias.