In acute myeloid leukaemia (AML), involvement of early progenitor cells may predict poor response to induction chemotherapy. We evaluated the involvement of early progenitor cells in two AML subtypes with a favourable prognosis [t(8;21) and t(15;17)], and a subtype with poor prognosis (monosomy 7). CD34+CD33- cells were isolated by fluorescence-activated cell sorting, grown in liquid medium followed by culture in semi-solid medium, and the colonies that were formed were analysed for the identifiable genetic markers. Two of 136 colonies from six t(8;21) AML patients expressed the AML1-ETO transcript, and all six patients achieved remission after induction. None of 192 colonies from five t(15;17) AML patients expressed the RARalpha-PML transcript and all achieved remission. In contrast, in three of 10 cases of monosomy 7 AML, colonies were positive for monosomy 7, and all three patients failed to enter remission. However, five of six evaluable patients with colonies negative for monosomy 7 entered remission. These data support the hypothesis that leukaemic involvement of early progenitor cells affects the response to induction chemotherapy.