Objective: We are studying associations between selected biomarkers and schizophrenia or bipolar disorder among military personnel. To assess potential diagnostic misclassification and to estimate the date of illness onset, we reviewed medical records for a subset of cases. Methods: Two psychiatrists independently reviewed 182 service medical records retrieved from the Department of Veterans Affairs. Data were evaluated for diagnostic concordance between database diagnoses and reviewers. Interreviewer variability was measured by using proportion of agreement and the κ statistic. Data were abstracted to estimate date of onset. Results: High levels of agreement existed between database diagnoses and reviewers (proportion, 94.7%; κ = 0.88) and between reviewers (proportion, 92.3%; κ = 0.87). The median time between illness onset and initiation of medical discharge was 1.6 and 1.1 years for schizophrenia and bipolar disorder, respectively. Conclusions: High levels of agreement between investigators and database diagnoses indicate that diagnostic misclassification is unlikely. Discharge procedure initiation date provides a suitable surrogate for disease onset.