Seventeen patients with acute myeloid leukaemia (AML) whose blasts co-expressed the T-cell associated CD7 antibody were identified among 160 consecutive AML cases. Fourteen had FAB defined AML according to morphocytochemical criteria, whereas three patients were classified as 'MO' on the basis of immunophenotype. The incidence of CD7 positively was particularly significant in the less differentiated subtypes M0 and M1 compared with other FAB groups (P less than 0.001). In all cases the myeloid determinants CD13 and/or CD33 were associated with CD7 expression. Other B-lymphoid (CD10, CD19) or T-lymphoid (CD2, surface and cytoplasmic CD3) markers were analysed and found to be negative. Five out of 15 cases examined were TdT+. Clonal rearrangements of the immunoglobulin heavy chain (IgH) and/or T-cell receptor (TcR) beta chain genes were identified in only three out of 13 cases. Among these, one out of five co-expressing TdT showed IgH rearrangement when analysed at the DNA level. Clinical features at presentation and response to induction therapy did not allow us to consider CD7+ AML patients as a distinct subgroup with prognostic significance. Our data indicate that CD7 expression is a common finding in immature AML, being generally found in the absence of other T-cell features. Rather than suggesting the occurrence of 'mixed leukaemia', such cases confirm a broader spectrum of CD7 reactivity and its possible identification of a particular subset of myeloid progenitors.