Cytogenetic analyses were carried out on peripheral blood and bone marrow cells of 31 chronic myeloid leukemia (CML) patients who presented with blastic, accelerated, or chronic phases. The percentage of cytoplasmic nonspecific cross-reacting antigen (cNCA, a marker of myelocytic differentiation)-containing cells was determined in the same blood or bone marrow samples. The patients were divided in two groups according to cytogenetic results: those with aberrations in addition to the Philadelphia chromosome (Ph1) and those with Ph1 only. Among the additional aberrations such changes, not typical of CML, were found: del(2)(p21), t(6;11)(q25;q23), and t(12;?)(p13;?). The survival time and the percentage of cNCA-positive cells of patients in blastic and accelerated phases were compared between the above-mentioned two groups of patients using the Student t test and the Kaplan-Meier estimator. The percentage of cNCA-positive cells was significantly lower and the survival time significantly shorter in the group of patients with additional aberrations. The probability of survival according to the Kaplan-Meier estimator was also lower for this group. These data suggest that the immunologically determined lower degree of maturity, that characterized cells bearing additional aberrations, coincides with and/or results in more rapid progression of the disease.