Background: Prostate cancer is a commonly studied outcome in administrative claims studies, but there is a dearth of validated case-identifying algorithms. The long-term development of the disease increases the difficulty in separating prevalent from incident prostate cancer. The purpose of this validation study was to assess the accuracy of a claims algorithm to identify incident prostate cancer among men in commercial and Medicare Advantage US health plans. Methods: We identified prostate cancer in claims as a prostate cancer diagnosis within 28 days after a prostate biopsy and compared case ascertainment in the claims with the gold standard results from the Georgia Comprehensive Cancer Registry (GCCR). Results: We identified 74,008 men from a large health plan claims database for possible linkage with GCCR. Among the 382 prostate cancer cases identified in claims, 312 were also identified in the GCCR (positive predictive value [PPV]=82%). Of the registry cases, 91% (95%CI=88, 94) were correctly identified in claims. Claims and registry diagnosis dates of prostate cancer matched exactly in 254/312 (81%) cases. Nearly half of the false positive cases also had claims for prostate cancer treatment. Thirteen (43%) false negative cases were classified as noncases by virtue of having a biopsy and diagnosis greater than 28 days apart as required by the algorithm. Compared to matches, false negative cases were older men with less aggressive prostate cancer. Conclusions: Our algorithm demonstrated a PPV of 82% with 92% sensitivity in ascertaining incident PC. Administrative health plan claims can be a valuable and accurate source to identify incident prostate cancer cases.