Twenty-three patients in various cytological subgroups of blast phase of Ph1-positive chronic myeloid leukaemia were investigated cytogenetically The correlation between prognosis and the cytological and cytogenetic features of blast cells was studied. The best prognosis was found in the myeloid group, followed by the lymphoid, myelomonocytic, megakaryoblastic and finally the promyelocytic groups, in this order. As regards the prognostic significance of the cytogenetic finding, the survival seemed to be the longest among patients with mosaic karyotypes i.e. when further aberrations occurred in some Ph1-positive cells. The course of the disease was the most favourable in the presence of i(17q), especially in patients whose cells had i(17q) as the only aberration. The most severe course of the disease could be found in patients with +8 and/or two Ph1. The results demonstrate the usefulness of chromosome investigations in distinguishing the different cytogenetic subgroups in terminal stages of CML, which also differ prognostically from each other.