We studied microsatellite instability (MSI) at the onset and during progression of 17 individuals with acute myelocytic leukaemia (AML). These included two cases of MO, eight with M1 and seven with M4, according to the FAB classification. The DNA from diagnostic, remission and relapsed stages of their disease was analysed at 69 loci. Two MSI were found in the diagnostic and remission phase paired samples (12%), and eight MSI were identified in six of the relapsed phase samples (35%). These results indicate that mismatch repair errors such as MSI are unimportant at the onset of AML, but might have importance during the progression of the disease.