Fluorescence in situ hybridization in combination with morphology (MGG/FISH) was used to detect minimal residual disease (MRD) in complete remission (CR) in 12 cases of acute leukaemia (six MDS-AML, five de novo AML, one pre-B ALL) with numerical chromosomal aberrations at diagnosis. Residual leukaemic cells could be detected in the remission bone marrows by MGG/FISH in five patients, whereas the other seven showed no abnormalities. All five patients with signs of MRD at CR relapsed in the bone marrow with 2-9 months, in contrast to two of seven with a normal finding by MGG/FISH at CR. In both these patients a second MGG/FISH analysis showed that a subpopulation of leukaemic blasts had reappeared, 4 and 5 months prior to the leukaemia becoming clinically overt. One patient suffered a CNS relapse, but without any evidence of bone marrow involvement. The remaining four patients with no evidence of MRD at CR were still in haematological remission at follow-up after 4, 11, 12 and 13 months, respectively. We conclude that MGG/FISH seems to be a clinically useful method to detect MRD in acute leukaemia and to predict relapses, particularly when repeat studies are performed during CR.