Acute promyelocytic leukemia represents 5-10% of acute myeloid leukemia cases (AML) recorded in the literature, occurring more frequently in young adults. It has a special clinical and biological behaviour when compared to the other forms of AML, being characterized by a particular morphology of blast cells (M3 in FAB classification), translocation of chromosomes 15;17, and disseminated intravascular coagulation at diagnosis or after the onset of chemotherapy. Within this AML subgroup there are 2 morphological subsets called the hypergranular promyelocytic leukemia and the hypogranular or variant form. We have studied clinical and laboratory aspects of 19 cases of AML M3 out of 217 AML cases, and observed a high incidence of failure to recognize the M3 variant form, although its diagnosis has been mainly based on cytomorphology. Only 4 out of 8 cases of the variant form received in our laboratory were correctly diagnosed, being the other 4 cases wrongly identified as the myelomonocytic subset of AML (M4). Immunophenotyping with monoclonal antibodies using CD2 and CD7 as T cell markers, CD10 and CD19 as B cell markers and CD33, CD13, CD14, CD15 and anti MPO as myeloid markers is a complementary diagnostic tool that permits solving difficult cases. It is important to classify AML correctly because of the special therapeutic and prognostic features of AML M3, which differently from other AML forms, has been successfully treated with cellular differentiating agents.