Surface markers were studied at first relapse in 66 cases of acute myeloid leukaemia (AML), using a panel of five monoclonal antibodies directed to CD13, CD14, CD15, CD33 and CD34 antigens. At time of relapse, there was increased expression of CD33 (P=0·002) and CD34 (P=0·0001), and decreased expression of CD13 (P=0·004) and CD15 (P=0·0001) antigens by comparison to initial diagnosis. There was no strict correlation with the FAB classification. However, CD13 and CD33 expression changes preferentially affected granulocytic leukaemias. At relapse, CD14 and CD34 were significantly more expressed in monocytic than in granulocytic AML (P=001 and 0·003 respectively). In a multivariate analysis, CD34 expression was associated with a low CR rate (P=0·001) and short survival (P=0·05), whereas CD15 expression was associated with long survival (P=0·0004). These results suggest that AML tends to relapse with a less differentiated phenotype than observed at diagnosis and that AML with less differentiated phenotype is of poor prognosis after first relapse, as also observed at diagnosis.