Clonal analysis of lymphocytes from patients with myelodysplastic syndrome (MDS) has been carried out using X-chromosome inactivation patterns detected by the probe M27β, and by polymerase chain reaction amplification of the immunoglobulin heavy chain gene hypervariable region, CDR3. Of 32 female patients heterozygous for M27β only seven (22%) demonstrate monoclonality of peripheral blood lymphocytes. 12 (37%) give unequivocal polyclonal results and the remaining cases give patterns of X-inactivation which cannot be interpreted either way. A study of 68 MDS patients showed five (7%) with a population of B-cells with a monoclonal rearrangement of CDR3 compared with none out of 60 normal individuals, none out of 15 with B-non Hodgkin lymphoma (B-NHL) in remission and 19 out of 25 (75%) of cases of B-chronic lymphocytic leukaemia (B-CLL). Monoclonal lymphocytes were found by both techniques in only two females with MDS. We conclude that the presence of polyclonal lymphocytes is a common finding in patients with MDS.