The Multidrug Resistance gene (MDR 1) is frequently expressed in acute myeloid leukaemia (AML). MDR 1 is associated with resistance to chemotherapy in vitro and with a poor response rate in AML. We have investigated the prognostic value of MDR 1 expression in relation to other patient characteristics with respect to response and survival. One hundred and thirty patients aged 0-88 years were treated for de novo AML with standard induction and consolidation chemotherapy. MDR 1 expression was determined by immunocytochemistry. Univariate and multivariate analyses were conducted to identify prognostic factors for reaching complete remission (CR) and for overall survival from diagnosis, in order to compare MDR 1 with known prognostic factors. Univariate analysis showed that higher MDR 1 expression was an adverse prognostic factor for CR (P<0.001), as was higher age (P<0.001) and unfavourable karyotype (P<0.01). These factors were also negative prognostic factors for overall survival (P<0.001, P<0.05 and P<0.005, respectively). In the multivariate analysis MDR 1 (P<0.001), higher age (P<0.001) and karyotype (P<0.01) were independent adverse prognostic factors for CR as well as for overall survival (P<0.001, P<0.005, P<0.001, respectively). Our data indicate that MDR 1 expression is a disease-related unfavourable prognostic factor which has a significant impact on complete remission and overall survival in AML. Analysis of MDR 1 may be used to determine prognosis in individual patients.