Abstract Background: Cancer Research Network (CRN) sites use administrative data to populate their Virtual Data Warehouse (VDW). However, information on VDW chemotherapy data validity is limited. The purpose of this study was to assess the validity of VDW chemotherapy data. Methods: This was a retrospective cohort study of women ≥18 years with incident, invasive breast cancer diagnosed between January 1999 and December 2007. Pharmacy and procedure chemotherapy data were extracted from each site's VDW. Random samples of 50 patients stratified on trastuzumab, anthracyclines, and no chemotherapy exposure was selected from each site for detailed chart abstraction. Weighted sensitivities and specificities of VDW compared with abstracted data were calculated. Cumulative doses calculated from VDW data were compared with doses obtained from the medical chart review. Results: The cohort included 13,497 patients with 6,456 (48%) chart review eligible. Patients in the sample (N = 400) had a mean age of 65 years. Trastuzumab, anthracycline, and other chemotherapy weighted sensitivities were 95%, 97%, and 100%, respectively; specificities were 99%, 99%, and 93%, respectively; positive predictive values were 96%, 99%, and 55%, respectively; and negative predictive values were 99%, 96%, and 100%. Trastuzumab and anthracyclines VDW mean doses were 873 and 386 mg, respectively, whereas abstracted mean doses were 1,734 and 369 mgs, respectively (R2 = 0.14, P < 0.01 and R2 = 0.05, P = 0.03, respectively). Conclusions: Sensitivities and specificities for CRN chemotherapy VDW data were high and dosages were correlated with chart information. Impact: The findings support the use of CRN data in evaluating chemotherapy exposures and related outcomes. Cancer Epidemiol Biomarkers Prev; 21(4); 673–80. ©2012 AACR.