To gain more insight into the understanding of myelodysplastic syndromes (MDS) as they occur in Thailand, a retrospective clinicopathologic analysis was conducted in patients (age > 15 years) diagnosed as MDS from January 1992 to December 1996 at the five major medical centers in various geographic regions of the country. The central reviewers independently examined the bone marrow and peripheral blood smears of all the patients and classify the disease according to the French-British-American (FAB) classification. There were a total of 117 eligible patients. The median age of the patients was 56 years (range 16-86). The male:female ratio was 1:1. Thirty-two percent of the patients were younger than 40 years. The frequency of the FAB subtypes was RA/RARS, 54.7; RAEB, 23.1; CMML, 9.4; and RAEB-T, 12.8%. Anemia was the most common symptom presenting in 84.6% of the patients. In the 34 patients in whom the cytogenetics in the bone marrow were analysed, 44.1% revealed abnormalities. Of these, monosomy 7 and trisomy 8 were the most common aberration, each being detected in 26.7% of the patients. Transfusions were the main therapeutic modality in 80% of the patients. Kaplan-Meier analysis revealed a 5 year survival rate of 29% for the whole group with a median survival of 24 months. Twenty-five percent of the patients had progressed to acute myelogenous leukemia (AML) with a median time to disease-progression of 23 months. The median survival for RA/RARS, RAEB, CMML and RAEB-T were 58.4, 19.9, 10.7 and 8.7 months, respectively (P < 0.001). The stepwise Cox regression analysis revealed the percentage of blasts in the bone marrow as the only parameter significantly associated with survival and disease progression. On comparison with data from other countries, the age of Thai patients with MDS is considerably lower than the western population but is comparable to other asian countries. The distribution of the FAB subtypes and the survival of the patients are similar. The major prognostic features, however, lie in the percentage of blasts in the bone marrow rather than the degree of the observed cytopenia.