We present cytogenetic data of 12 new cases of uveal melanoma. One case showed a hypotetraploid chromosome number; the others were near-diploid. Underrepresentation or monosomy of chromosome 3 as well as multiplication of chromosomes 8, 8q, or regions of 8q was found in five cases. Both anomalies were observed in three cases. In two of these, multiplication of 8q resulted from isochromosome formation. The smallest multiplicated region seemed to be 8q23-24-->qter. Chromosome 6 anomalies were found in five cases without monosomy 3. Aberrations of chromosome 21 were seen in five cases and found to be the only cytogenetic change in two of them. These anomalies may define a subgroup of uveal melanoma. An anomaly of chromosome 1 was found in three cases. Anomalies of other chromosomes were detected in at most two cases each. Nonclonal centromeric or telomeric associations were seen in one case and different nonclonal ring chromosomes in another. Underrepresentation of chromosome 3 was seen exclusively in tumors with ciliary body involvement, while anomalies of chromosome 6 were detected only in pure choroideal melanomas. A review of the published cytogenetic data of untreated uveal melanoma supports our finding that monosomy 3 is frequently associated with multiplication of 8q but rarely with anomalies of chromosome 6, whereas the opposite is true for tumors with disomy 3, and that isochromosomes are closely associated with monosomy 3. Thus, cells with monosomy 3 seem to predispose to isochromosome formation.