Context.-Fluorescence in situ hybridization (FISH) analyses on tumor sections and on isolated nuclei showed that even low numbers of cells with monosomy of chromosome 3 adversely affected survival. Objective.-To determine what percentage of uveal melanoma cells with monosomy of chromosome 3 influences patient mortality. Design.-To determine the presence of monosomy 3, karyotyping and FISH on cultured cells and FISH on isolated nuclei were performed on 50 primary uveal melanomas. Clinical and pathologic prognostic factors were assessed and compared with 5-year survival data. Analyses were performed using Cox proportional hazards test, log-rank analysis, sensitivity, specificity, and positive and negative likelihood ratios. Results.-Combined karyotyping and FISH on cultured cells showed monosomy 3 in 19 of 50 cases (38%), whereas determination of the monosomy 3 status by FISH on isolated nuclei with a threshold of 5% assigned 31 of 50 cases (62%) to the monosomy-3 category. When monosomy 3 on isolated nuclei with a cutoff value of 5% was used, a significant difference in 5-year survival was present (hazard ratio, 15.5; P= .007), indicating that monosomy 3 in greater than 5% of tumor cells is related to death due to metastases. Conclusion.-In uveal melanoma, the presence of greater than 5% of cells with monosomy3, as determined by FISH on isolated nuclei, is associated with the development of metastases within 5 years after enucleation.