The clinical significance of the expression of CD7 antigen on the blasts of 207 consecutive patients with de novo acute myeloid leukemia (AML) was evaluated. For this purpose, fifty-three CD7+ patients (23 females and 30 males; mean age 52 years) were analyzed and classified into the following subtypes according to French-American-British (FAB) classification: 7 M0, 13 M1, 9 M2, 1 M3, 9 M4, 14 M5. Immunophenotypic studies were carried out by flow cytometry and blast cells were selected on the basis of forward light scatter gating and pan-myeloid marker, either CD13 or CD33. All the CD7+ patients were negative for surface CD3 and T-cell-receptor (TCR) molecules. We found no correlation between CD7 expression and sex, age, hepatosplenomegaly and/or central nervous system involvement. The immaturity of CD7+ leukemic cells was supported by the high expression of CD34 (P = 0.001). CD7 positivity was significantly associated with a white blood cell count (WBC) greater than 100 x 10(9)/L (P = 0.003). P-Glycoprotein (P-170) expression was also evaluated in 135 patients by a flow-cytometric assay: there was a close relationship between CD7 and P-170 positivity (P < 0.001). For remission induction, all patients received therapeutic regimens routinely used for AML. The complete remission (CR) rate was significantly lower in CD7+ cases (32% vs 74%, P = 0.001). The overall survival and disease free survival rate of CD7+ AML was lower than those of CD7- patients (P < 0.001 and = 0.002, respectively). CD7+ AML with coexpression of CD14 had a particularly unfavourable response and prognosis in comparison with CD7+ patients without CD14.(ABSTRACT TRUNCATED AT 250 WORDS)