The immunocytochemical detection of multidrug-resistance (MDR) associated P-glycoprotein (P-170) was longitudinally performed on bone marrow smears from 32 responsive patients with acute leukaemia in the different phases of the disease (at diagnosis, in complete remission, at relapse) by means of APAAP technique and monoclonal antibody C219. The whole group of eight patients with presence of P-170 positive cells while in complete remission rapidly relapsed with a high proportion of blasts showing MDR phenotype; they were resistant to further treatments. Twelve out of 24 subjects without cells with MDR phenotype in complete remission remained in this condition, six had a responsive relapse (without significant expression of P-170 in 5/6 patients) and six a resistant relapse. Four patients of this last group significantly expressed P-170. Our data indicate that the detection of scattered P-170 positive cells during complete remission might identify a subset of leukaemic patients with high risk of early and resistant relapse.