We report a cytogenetic study of six Merkel cell carcinomas (MCC) in which rearrangement of chromosome 1 was noted in four cases: two cases were trisomic, in one case there was a reciprocal translocation between chromosomes 1 and 5 [t(1;5)(p36;p13)], and in the fourth case all cells had a normal chromosome 1 and three derivatives, a del(1)(p22) and del(1)(q21), and a translocation involving material of unknown origin to the long arm, t(1;?)(q21;?). Four cases demonstrated loss of chromosome 13; in two of these, both copies were lost, and the survival for these two patients was much longer than is common for MCC patients. Partial trisomy of chromosome 11 was noted in two cases, and two patients demonstrated loss of chromosome 22 in all cells examined. Although no consistent chromosome change was noted in our cases, our data and those of previously published reports, show that abnormalities of chromosomes 1, 11, and 13 occur in 30-47% of cytogenetic reports of this rare malignancy.