Bone marrow karyotype, survival time, and the rate of progression to leukaemia were studied in 111 unselected patients with primary myelodysplastic syndromes. The 49 patients (44%) with clonal chromosome aberrations had survival time (median 29 months) similar to that found in the 62 patients with normal bone marrow karyotype (24 months, p greater than 0.10). The presence of multiple (greater than 2) abnormalities (17 patients) was strongly associated with poor prognosis, with a median survival of only 7 months (p less than 0.001). Prognostic information could be attributed to 2 specific abnormalities, del(5q) and -7: Presence of del(5q) as the sole anomaly was associated with long survival (36+ months), whereas monosomy 7 was a bad prognostic sign (6 months). The risk for leukaemia development correlated neither with the number of chromosome abnormalities nor with any particular anomaly. Our findings demonstrate the prognostic importance of quantifying the complexity of bone marrow chromosome changes. They also emphasize that different specific abnormalities convey widely different prognostic information in primary myelodysplastic syndromes.