72 patients were diagnosed as suffering from myelodysplastic syndromes (MDS) according to the FAB classification: 16 patients with refractory anaemia (RA), 11 patients with acquired idiopathic sideroblastic anaemia (AISA), 14 patients with refractory anaemia with an excess of blast cells (RAEB), 7 patients with RAEB in transformation (RAEB/t) and 24 patients with chronic myelomonocytic leukaemia (CMML). The duration of the preleukaemic phase was between 2 and 189 months (median: 15 months); RAEB in transformation and CMML showed a median phase of less than 12 months. Transformation into acute leukaemia (AL) occurred in 46 patients (64%). Of the clinical signs only thrombocytopenia was a significant poor prognostic factor (p less than 0.01). Cytogenetic studies were made in 31 patients. 14 had clonal aneuploidy: these patients had a higher risk of AL, but not a significantly shorter preleukaemic phase (p greater than 0.1). Stem cell cultures (CFUc) were carried out in 31 patients. Patients without colony growth or only cluster growth showed a high incidence (10/11 and 8/8) of transformation into AL; preleukaemic phases were significantly shorter than in patients with normal colony growth or cluster + colony growth in all FAB subgroups (p less than 0.001). The bone marrow blast cell count was indirectly proportional to the duration of the preleukaemic phase: thrombocytopenia, cytogenic aberrations and failure of in vitro colony growth are additional poor prognostic factors in MDS.