For men with prostate cancer, there is substantial variation in the use of conservative management, such as active surveillance. Commercial prices, which vary across urology practices, may afford incentives that foster physician behaviors associated with utilization. Such behaviors may "spillover" to the Medicare population and affect quality. This study evaluated the effects of practice-level commercial prices on health care utilization and quality in men with prostate cancer insured by traditional Medicare. From a 20% Medicare sample, the authors identified men with newly diagnosed prostate cancer between 2014-2019 (n=44,653). Using commercial payments from the MarketScan database, they developed a practice-level commercial price index (ratio of commercial prices to Medicare prices). They examined the association of the price index with price standardized spending, overtreatment (treatment among those with >50% noncancer mortality within 10 years), and underuse of diagnostic testing in active surveillance (at least one prostate-specific antigen test and one confirmatory test-MRI, prostate biopsy, genomic test-within 12 months of diagnosis). Practice-level commercial price indices varied from 1.34 (134% of Medicare prices), for practices in the bottom decile, to 3.00, for practices in the top decile. Increasing price index was associated with lower odds of overtreatment (odds ratio, 0.86; 95% confidence interval, 0.76-0.97; p=.01), but not price standardized spending or underuse of diagnostic testing in active surveillance. Commercial prices vary markedly across urology practices. Among newly diagnosed men with traditional Medicare, those managed by practices with higher commercial price indices had lower odds of overtreatment, suggesting improved prostate cancer care quality.