Out of 412 adult patients classified as aplastic anaemia in a co-operative study group, 22 developed acute leukaemia from 3 months to 5 years after the initial diagnosis. In 21 of these, retrospective analysis elicited one or several clinical and laboratory data which are uncommon in aplastic anaemia but common in pre-leukaemic refractory anaemia. Indeed, one half of these patients should have been classified as pre-leukaemic, and in the remaining half cell kinetics investigations (analysis of radioactive iron renewal, stem cell cultures and 3H-TdR autoradiography of bone marrow), as well as cytogenetic studies, might have led to a more correct classification. Only one patient, who initially had none of the criteria of pre-leukaemic state, responded to androgen treatment with a 4-year complete remission but died on the 5th year of acute myeloblastic leukaemia. It appears from this retrospective analysis that when all signs of pre-leukaemia have been excluded by thorough investigation, aplastic anaemia is never, or only exceptionally (1 out of 412 cases), a pre-leukaemic condition.