DNA index (DI) determined by flow cytometry and karyotype determined by conventional methods were obtained on bone marrow samples from 43 haematologically normal subjects and 54 patients with myelodysplastic syndrome (MDS). Twenty one patients had a clonal karyotype abnormality but an additional five had a DI outside the normal range, showing evidence of aneuploidy that was not available from chromsome preparations. When patients were grouped into those with excess chromosomal material, those with diploid karyotypes, and those with a loss of chromosomal material, there was a significant difference among the mean DIs of each group, normal subjects being different from all patient groups. In these patients DI measurements were of value when carried out together with conventional chromsomal analysis in gaining the maximum amount of genetic information when a satisfactory karyotype might not be available or where failure of an abnormal cell population to proliferate might give an incomplete cytogenetic picture. The contribution of non-clonal chromsome loss to the DI is probably significant but has not been quantitated.