To compare information from self-report and electronic medical records for four common comorbidities (diabetes, hypertension, myocardial infarction, and other heart diseases). We pooled data from two multiethnic studies (one case-control and one survivor cohort) enrolling 1,936 women diagnosed with breast cancer, who were members of Kaiser Permanente Northern California. Concordance varied by comorbidity; kappa values ranged from 0.50 for other heart diseases to 0.87 for diabetes. Sensitivities for comorbidities from self-report versus medical record were similar for racial/ethnic minorities and non-Hispanic Whites, and did not vary by age, neighborhood socioeconomic status, or education. Women with a longer history of comorbidity or who took medications for the comorbidity were more likely to report the condition. Hazard ratios for all-cause mortality were not consistently affected by source of comorbidity information; the hazard ratio was lower for diabetes, but higher for the other comorbidities when medical record versus self-report was used. Model fit was better when the medical record versus self-reported data were used. Comorbidities are increasingly recognized to influence the survival of patients with breast or other cancers. Potential effects of misclassification of comorbidity status should be considered in the interpretation of research results.