Chronic myelocytic leukemia is a particular subtype of leukemia characterized by increased myeloid precursor cells. It has been associated with the presence of the Philadelphia chromosome, described by Nowel and Hungerford in 1960, as a deletion of part of the long arm of a G group chromosome, the 22 chromosome. The present work reports the chromosomal abnormalities observed in 39 patients with chronic myelocytic leukemia, studied at the Genetic Unit, in the Faculty of Medicine of Zulia University, during the period from 1987 to 1991. Sixty per cent of the patients showed different abnormalities, such as 8 trisomy, t (8;22), and in the remaining 15%, no chromosomal changes were detected. The patients with t (9;22) as the only abnormality, had less relapses and longer survival. The clinical course of 50% of the patients with normal karyotype was similar to those with t (9;22) as the only abnormality; the other 50% had an accelerated course with frequent relapses and early death. The present findings confirm that the presence of the Philadelphia chromosome as the only karyotypical abnormality, is indicative of better prognosis, and its association with other chromosomal changes predicts a more accelerated course that will probably require a more aggressive treatment.