The prognostic importance of pretreatment clinical and laboratory features was investigated in a group of 243 patients with Philadelphia chromosome positive chronic phase chronic myeloid leukemia from 1977-1995. Chemotherapy consisted of busulfan before 1993 or hydroxyurea after 1993. The overall median survival from diagnosis was 28 months. The mean age of the patients was 38 years, about 10 years below that of Western populations. Univariate analysis identified 4 poor prognostic features: thrombocytopenia, more than 5% peripheral blasts, more than 5% erythroid precursors and less than 7 g/dl of hemoglobin. The median survival times of patients with these 4 risk factors were 5, 11, 11 and 12 months respectively. Multivariate analysis only identified 2 significant prognostic features: thrombocytopenia and more than 5% peripheral blasts. Splenomegaly of more than 10 cm, basophilia and leukocytosis were associated with a shorter median survival but was not statistically significant. A risk scoring system was developed and used to classify patients into low, intermediate and high risk groups at 30.9%, 30.2% and 38.8% respectively. The median survival time according to the low, intermediate and high risk group was observed at 60, 27 and 14 months respectively. Prognostic factors for Thai patients with chronic myeloid leukemia have both similarities and differences with previously observed factors but the median patient survival time is shorter.