Costimulatory molecules such as lymphocyte function-associated antigen (LFA)-1 (CD11a), LFA-3 (CD58), intercellular adhesion molecule (ICAM)-1 (CD54), neuronal cell adhesion molecule (NCAM) (CD56), B7-1 (CD80), or B7-2 (CD86) are important regulatory elements in healthy immunological cascades, but their role in acute myeloid leukemia (AML) has only been rarely investigated. We studied their expression on mononuclear bone marrow (BM) cells from 105 patients with AML at initial diagnosis and evaluated their prognostic significance. Fluorescence-activated cell sorter (FACS) analyses were performed using antibodies directly conjugated with fluorescein. A BM sample was considered positive if more than 20% of the cells in the blast containing gate expressed the respective marker. The surface expression of CD11a (27 of 29 cases positive with an average of 71% positive blasts; 27(+)/29, 71%), CD54 (23(+)/33, 37%), CD56 (24(+)/93, 20%), CD58 (29(+)/29, 95%), CD80 (13(+)/28, 30%), and CD86 (19(+)/29, 39%) was measured. The expression of these markers in different French-American-British (FAB) classification types (M0-M5) was heterogeneous, except for CD56, which showed a higher proportion of positive cells in monocytic subtypes of AML. In addition, cases with a "poor risk" karyotype as well as patients succumbing to "early death" after double induction therapy according to the AML Cooperative Group (CG) protocol were characterized by a high expression of CD56. Relapse-free survival analyses demonstrated that patients with more than 8% CD56(+) cells in the BM relapsed significantly sooner. CD54 was preferentially expressed in AML M4(eo) and in addition in "favorable" cytogenetic risk groups and in cases that had responded to AML-CG therapy. Only very high proportions (>60%) of CD54(+) cells were associated with a lower probability for relapse-free survival. CD80 and CD86 expressions were similar in all FAB types. Patients who had responded to AML-CG therapy showed higher CD80 proportions and lower CD86 proportions compared to the "nonresponder" group. Whereas cases with more than 15% CD80(+) cells had a significantly lower probability for relapse-free survival, only cases with more than 65% CD86(+) were characterized by a significantly lower probability for relapse-free survival. Expression profiles of CD11a and CD58 were not associated with specific FAB types or prognostically relevant groups. We can conclude: (1) Expression of costimulatory molecules in AML is very variable. This reflects the great diversity of immunophenotypes in AML. (2) CD56 is mainly expressed in monocytic subtypes of AML. CD56(+) subtypes of AML seem to be a separate entity with a worse prognosis independent of the karyotype. (3) High expression of some costimulatory molecules correlates with a worse prognosis concerning relapse-free survival times.