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Cancer registries usually exclude nonmelanoma skin cancers (NMSC), despite the large population affected. Health maintenance organization (HMO) and health system administrative databases could be used as sampling frames for ascertaining NMSC. NMSC patients diagnosed between January 1, 1988, and December 31, 2007, from such defined US populations were identified by using 3 algorithms: NMSC International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes, NMSC treatment Current Procedural Terminology (CPT) codes, or both codes. A subset of charts was reviewed to verify NMSC diagnosis, including all records from HMO-enrollee members in 2007. Positive predictive values for NMSC ascertainment were calculated. Analyses of data from 1988-2007 ascertained 11,742 NMSC patients. A random sample of 965 cases was selected for chart review, and NMSCs were validated in 47.0% of ICD-9-CM-identified patients, 73.4% of CPT-identified patients, and 94.9% identified with both codes. All charts from HMO-health plan enrollees in 2007 were reviewed (n = 1,116). Cases of NMSC were confirmed in 96.5% of ICD-9-CM-identified patients, 98.3% of CPT-identified patients, and 98.7% identified with both codes. HMO administrative data can be used to ascertain NMSC with high positive predictive values with either ICD-9-CM or CPT code, but both codes may be necessary among non-HMO patient populations.