We describe the occurrence of an unusual mode of relapse in six of 24 patients who presented with de novo acute myeloid leukaemia (AML) associated with trilineage myelodysplasia (TMDS). After the induction of complete remission (CR) by intensive chemotherapy in five patients and following bone marrow transplantation (BMT) in one, the myelodysplastic state, but not overt AML, recurred. Relapse of myelodysplasia occurred at a median of 147 weeks (50-520) from presentation and in two instances was followed a year later by AML. In five cases, myelodysplastic relapse was treated with low-dose cytosine arabinoside given alone or with other chemotherapeutic agents. Three patients remain in CR after 1, 2 and 5 years. The reappearance of myelodysplastic features in these six patients was strongly correlated with the presence of TMDS at presentation of the AML. It was not observed once in the 136 AML patients, treated similarly, who did not have associated TMDS at presentation (P less than 0.001). Thus, relapse with myelodysplasia is not an effect of chemotherapy as has been previously postulated.