We report 2 cases of hypothyroidism in whom the associated haematological disturbances (macrocytic anaemia in both cases, mild leukopenia and thrombo-penia in one patient) failed to be corrected by thyroid hormone replacement therapy. Further investigations led to the diagnosis of acute myeloid leukaemia (AML) in the first patient and myelodysplasia syndrome (MDS) in the other. The reasons of the delayed diagnosis and the possible mechanisms explaining this rare combination are discussed. The hypothesis of a purely coincidental association seems most likely.