This paper reports the results of cytogenetic studies in a consecutive series of 64 patients with polycythemia vera, 57 of whom could be followed prospectively. The median length of the cytogenetic observation time was 93 months (range, 24 to 224 months) after diagnosis. Clonal chromosome abnormalities were observed initially in 11 patients (17%) and later during the course of the disease in another 20 patients. An abnormal karyotype was found in 71% to 80% of the patients who were examined after the development of myeloid metaplasia, myelofibrosis, or leukemia. Patients treated with myelosuppressive agents showed a significantly greater risk of chromosome abnormalities developing than did patients who had been phlebotomized. Acute leukemia developed in eight patients, all of whom had been treated with myelosuppressive agents. A chromosome abnormality preceded the leukemia in only two of the patients. The initial presence of an abnormal karyotype did not predict a greater risk of development of leukemia. No consistent relationship was demonstrated between the occurrence of chromosome abnormalities and the development of myeloid metaplasia and/or myelofibrosis, which was observed in 42% of the patients. The chromosome abnormalities followed a nonrandom pattern, and those most frequently observed were trisomies for 1 q, 8, 9, or 9p and deletion of 20q. Deletions seem to be common and were found in 14 patients.