Forty-two Ph1-positive cases of chronic myelocytic leukemia (CML) were examined with chromosomal banding techniques. Thirty-seven of these cases had the "standard" type of Ph1 translocation between chromosomes No. 9 and No. 22 [t(9;22)(q34;q11)] in the Ph1-positive marrow cells; 5 cases had unusual types of Ph1 translocation. Of the 37 cases, 21 had additional numerical and/or structural chromosomal changes, 2 had a missing Y chromosome, and 1 had an extra Ph1 in the Ph1-positive cells. In the 5 cases with unusual types of Ph1 translocation, chromosomes No. 2, No. 9 No. 10, and No. 13 were involved. The clinical picture in these 5 patients did not differ materially from that of the other Ph1-positive patients with CML, probably indicating that the recipient chromosome, with which the translocation from No. 22 takes place, does not play a crucial role in the course of the CML. In the 21 cases with abnormal karyotypes, nonrandom chromosomal changes were observed. Most of the changes were related to events occurring at the centromeric region. The prognosis of cases with only an extra No. 8 or Ph1 appears to be better than that for cases with an iso-17q [I(17a)] chromosome or other extra chromosomes. The presence of the Ph1 (delected No. 22) in every case points to the essentiality of this karyotypic findings in the diagnosis of CML and possibly in the genesis of the disease.