In order to assess the relative importance of the clinicohematological features most commonly associated with the accelerated phase (AP) of chronic myeloid leukemia (CML) in 175 consecutive patients, 12 variables generally considered as indicating AP were analyzed for their predictive value for blast crisis (BC) appearance in less than 1 yr. At the time of analysis, 118 patients had died and 104 had developed BC. At univariate study, 6 features were associated with a significantly higher BC-probability: poor performance status (ECOG score > or = 2), unexplained fever/sweats, severe bone pain, progressive splenomegaly despite adequate therapy, blood basophils (> or = 20%) and peripheral blasts (6-12%). At logistic regression, only bone pain and blood blasts (6-12%) retained their prognostic importance; the relative risk of unexplained fever/sweats and progressive splenomegaly was also clinically relevant. One-year BC-probability from the appearance of 1 or more of the above features was 77.3% (95% CI: 66-86.6) and 100% since all 4 were observed. Finally, at least 1 of the 4 features was present prior to death in 6 of 7 patients dying from CML-related causes while not in BC. AP can be defined by the appearance along CML evolution of 1 or more of the 4 above-mentioned clinicohematological features.