CD56 antigen expression has been reported in several hematologic malignancies. In acute myeloid leukemia (AML)M2 with t(8;21) and acute promyelocytic leukemia (APL) it has been found to be consistently associated with an unfavorable prognosis, whereas in other AML subtypes its role remains uncertain. We investigated CD56 expression in a cohort of AML patients in order to assess its frequency and prognostic relevance.Immunophenotypic analysis including that of CD56 antigen was available for 171 consecutive AML patients (139 with AML and 32 with APL), enrolled between December 1995 and December 1999 at a single institution. A sample of fresh bone marrow cells taken at diagnosis was recorded as positive when at least 20% of the cells double-stained with specific monoclonal antibodies against CD56 and CD33 antigens.CD56 positivity was demonstrated in 37 cases (21.6%). Its frequency was lower in M4 (6%) and higher in M5 (37%). The median percentage for CD56+ blasts was 56% (range 21-99%). CD56 positivity did not correlate with age, sex, blast count, favorable or unfavorable cytogenetics at diagnosis, nor did it influence the outcome in terms of complete remission (CR) duration (606 vs. 417 days, p=n.s.) or overall survival (OS) (210 vs. 277 days, p= n.s.). In the APL subgroup a significant difference in relapse rate was found at 3 years (71.4% in the CD56 positive group vs. 12% in the CD56 negative group, p=0.005).Our data confirm that CD56 positivity in APL patients at diagnosis is associated with a worse prognosis, suggesting that close molecular monitoring is necessary in CD56 positive APL patients. In contrast, the prognostic role of CD56 remains uncertain in the other AML subtypes.