We report 72 blastic crises (BC), occurring in 238 Ph+ chronic myeloid leukemia (CML) patients treated in chronic phase (CP) with alpha-interferon (IFN) for a median time of 51 months (range 7-96). The 238 patients were grouped by Sokal's risk at diagnosis in low- (LR), intermediate- (IR) and high-risk (HR), and by CP treatment. Group 1: 160 patients (57% LR, 31% IR, 12% HR) given IFN alone in early CP. Group 2: 31 patients (65% LR, 32% IR, 3% HR) given IFN alone in late CP. Group 3: 23 patients (78% LR, 22% IR) given IFN before and after autologous stem cell transplantation (ASCT). Group 4: 24 patients (83% LR, 17% IR) given IFN after ASCT. Of the 72 BC, 52 (72%) were myeloid (My), and 20 (28%) lymphoid (Ly). Overall BC incidence was similar in all CP treatment groups, although with a prevalence of Ly BC in groups 3 + 4 vs groups 1 + 2, (p = NS); the incidence of BC was higher in HR patients (P = NS), but on the whole it was lower than expected on the basis of historical controls. Lymphoid BC was more frequent in LR than in IR + HR patients (P < 0.05), and was more frequent in responders to IFN, than in non-responders (P < 0.05). In conclusion, a subset of patients with low risk at diagnosis, better response to IFN and proneness to evolve into Ly BC can be identified. The role played by IFN in this context remains to be defined.