The majority of patients with acute myelogenous leukaemia (AML) and 
myelodysplastic syndromes (MDS) relapse, especially those with unfavourable 
cytogenetics. This study was designed to investigate the presence and frequency 
of minimal residual disease (MRD) in patients with AML or MDS (n=35) and 
numerical abnormalities of chromosomes 6, 7, 8, 9, 10, 17 and 18 in clinical 
remission by using a combination of fluorescence activated cell sorting (FACS), 
fluorescence in-situ hybridization (FISH) and labelling with bromodeoxyuridine 
(BUdR). The technique enables the detection of as few as three leukaemic cells 
in 10(5) normal cells. MRD was detected in 33/35 patients in complete remission 
(CR). 16 patients relapsed (8/11 with monosomy 7, 4/17 with trisomy 8, and 4/7 
with other cytogenetic abnormalities) after a median of 4.8 months (range 3-13). 
Levels of MRD (P=0.007) and proliferation index (P=0.011) were significantly 
higher in patients with monosomy 7 than in patients with trisomy 8 or other 
cytogenetic abnormalities. The percentage of cells in S-phase, the number of 
abnormal cells and cytogenetic class were related to time to relapse (P=0.001) 
with S-phase being the single most important prognostic factor (P=0.0001). We 
conclude that the combination of FACS/FISH/BUdR, which determines the number, 
phenotype and proliferation rate of very rare leukaemic cells in patients with 
AML or MDS in clinical remission, provides information that is useful in the 
identification of patients with high and low likelihood of relapse.