As it has been shown that alpha-interferon (alphaIFN) treatment modifies the survival of chronic myeloid leukaemia (CML) patients in comparison with conventional chemotherapy, a new prognostic score was devised with the aim of providing a treatment-adapted risk evaluation. We have tested the new prognostic score (the Euro score) in an independent series of 272 patients less than 56 years old with previously untreated, chronic phase, Philadelphia (Ph)-positive CML who had been assigned prospectively to alphaIFN treatment between 1989 and 1991. The Sokal score system was used as a reference. The new Euro score predicted the response to alphaIFN as the Sokal score. The median survival of low-risk, intermediate-risk and high-risk patients was similar using the Euro score (105, 65 and 45 months) and Sokal score (105, 76 and 45 months) but, by multivariate analysis, the Euro was more potent than Sokal for predicting survival time. The new Euro score identified more low-risk cases (59% vs. 48%) and fewer high-risk cases (9% vs. 23%) than the Sokal score. The main differences between the Euro and Sokal scores concerned age (it is more important in the Euro than in Sokal), spleen size and the percentage of myeloblasts in peripheral blood (more important in Sokal than in Euro). We conclude that the new Euro score marks an improvement in the prognostic evaluation of CML treated with alphaIFN. By comparison with the Sokal score, the Euro was more potent and identified more low-risk patients but left only a small number of cases in the high-risk group.