The chromosomes of bone marrow cells from ten patients with polycythemia vera (PV) were identified by Q-, G-, and C-banding techniques. Four of the patients had received no treatment with cytotoxic drugs, while three had received 32P only and the other three, in addition, had received busulfan or busulfan and procarbazine. One 73- yr-old male patient treated with venesection only for 4 yr lacked the Y chromosome and had a deletion of the long arm of chromosome 20 (20q-) in all cells investigated. One of the other three patients who had received no drugs had a chromosome abnormality, but only in 1 of 19 identifiable metaphases. However, the abnormality was the same (+9) as the most common one in treated patients. In the group of treated patients, an extra chromosome 9 (+9) was found in three patients, an extra chromosome 8 (+8) in one, and a deletion of the long arm of one chromosome 20 (20q-) in one patient. Multiple aberrations in addition to the extra chromosome 9 were found in one patient in whom the disease had transformed into acute myeloblastic leukemia. The finding of identical chromosomal aberrations (20q- and +9, respectively) in two patients who had received no drugs and in four patients who had received 32P and busulfan or procarbazine favors the view that these aberrations are specifically associated with the disease and not induced by the drugs. With the exception of the patient with acute myeloblastic leukemia, all other patients are alive 1–11 mo after chromosome analyses and 1–229 mo after diagnosis.