A series of 94 patients presenting primary refractory anaemia with excess of blasts in transformation or acute myeloid leukaemia occurring after a myelodysplastic stage was submitted to retrospective cytogenetic analysis by the Groupe Français de Cytogénétique Hématologique. The aim of this collaborative study was to analyze the patterns of chromosome abnormalities appearing in primary myelodysplastic syndromes (MDS) in leukaemic transformation. As previously described in the literature, the most common chromosome aberrations involved del(5q), -7, +8, 17, 11, 12p and del(20q), while abnormalities of chromosome 17p were more frequently detected during the leukaemic transformation of MDS. The translocations t(2;3) (p22-23;q26-28) and whole arm t(17;18) were confirmed to be nonrandom events in these myeloid disorders.