Based on a 6 1/2-year study of 284 consecutive adult patients with primary myelodysplastic syndrome (MDS) and and acute myelogenous leukaemia (AML), we have found that refined chromosome analysis can be used as an independent prognostic indicator in the great majority of patients with MDS and AML. In MDS, the FAB subtype was also found to have prognostic value and this was enhanced when the chromosomal findings were taken into consideration. In AML, the age of the patient correlated more closely with the chromosomal changes in predicting prognosis in most patients than did the FAB classification. Previously we reported that refined chromosome analysis of bone marrow specimens from 161 adult patients with primary or non-therapy related MDS and AML identified three prognostic chromosomal categories in each disease, representing 40% of all patients (Yunis et al, 1984, 1986). By extending our study to 284 patients, as well as a longer follow-up, it was possible to determine the prognostic implications of two additional chromosomal categories in MDS and five in AML. Since 73% of all patients are now represented in well-defined chromosomal subgroups with prognostic significance, refined chromosome analysis emerges as a tool that could have considerable impact in protocols.