The blast crisis in chronic myeloid leukaemia (CML) is morphologically well defined. The aim of this report was to analyze whether immunophenotyping can divide CML blast crises into subtypes as is done in acute de novo leukaemia, and whether a specific CML pattern exists. Between 1991 and 1997 we reevaluated all the immunophenotypes of patients with CML blast crises with special regard to immunological subclassification, expression of CD34, presence of aberrant markers and number of immunological clusters. Twenty-nine CML blast crises were analyzed. Seventeen were myeloid, 11 lymphoid and one biphenotypic. The blast crises were divided into subtypes as de novo acute leukaemias: of the 17 myeloid blast crises 6 were undifferentiated, 5 differentiated and 6 had monocytic differentiation. In the lymphoid blast crises there were no pro-B, 8 common-B and 3 mature-B. No T-lymphoid blast crises were observed. In 26/29 analyses (90%) CD34 was expressed in the blasts. In 17/29 analyses (59%) one or two aberrant markers were found. In summary, immunophenotyping is important in distinguishing between myeloid and lymphoid blast crises. A subclassification, as in acute leukaemias, is possible. We found no specific immunophenotypic CML pattern. A study directly comparing immunophenotyping of CML blast crises with acute de novo leukaemia is planned.