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Purpose: Gain of chromosome 8q has been associated with poor prognosis in uveal melanoma (UM), and an increase in the absolute number of 8q-copies correlated with an even shorter survival. Splicing factor 3b subunit 1 ( SF3B1 )-mutated ( SF3B1 MUT ) tumors display structural chromosomal anomalies and frequently show a partial gain of chromosome 8qter. A recent subset of SF3B1 MUT UM with early-onset metastases has been identified, prompting the investigation of the relationship between survival, 8q gain, and SF3B1 MUT UM., Design: Retrospective cohort study., Subjects: Patients diagnosed with UM who underwent enucleation or received a biopsy at the Erasmus MC Cancer Institute or the Rotterdam Eye Hospital, The Netherlands were included., Methods: Fifty-nine patients with SF3B1 MUT tumors and 211 patients with BRCA1 associated protein 1 ( BAP1 )-mutated ( BAP1 MUT ) tumors were included in this study. Copy number status and gene expression were assessed using either a single nucleotide polymorphism array, fluorescence in situ hybridization, and karyotyping, or a combination of these techniques. Disease-free survival was determined and a cut-off of 60 months was used to define early-onset metastatic disease., Main Outcome Measures: Disease-free survival., Results: Forty-eight patients with SF3B1 MUT UM (81%) had chromosome 8q gain (3 copies, 78%; 4 copies, 22%). Kaplan-Meier analysis of SF3B1 MUT UM did not indicate a difference in survival in patients with or without gain of 8q ( P  = 0.99). Furthermore, the number of 8q copies was not associated with survival when comparing early ( P  = 0.97) versus late ( P  = 0.23) metastases group. In contrast, the presence of 8q gain (86%) was correlated with a decreased survival in BAP1 MUT UM ( P  = 0.013)., Conclusions: We did not find a correlation between 8q gain and early-onset metastasis in SF3B1 MUT tumors. Gain of 8q has no additional predictive value in SF3B1 MUT tumors. In contrast, 8q gain is predictive of a worse prognosis in patients with BAP1 MUT tumors. Thus, gain of chromosome 8q has additional predictive value for BAP1 MUT tumors, but not for SF3B1 MUT tumors., Financial Disclosures: The author(s) have no proprietary or commercial interest in any materials discussed in this article., (© 2023 by the American Academy of Ophthalmology.)