Comorbidity generally is measured by medical record abstraction, which is expensive and often impractical. The aim of this study was to assess the reproducibility and validity of a comorbidity questionnaire. The authors developed a brief comorbidity questionnaire that included items corresponding to each element of the medical record-based Charlson index. The questionnaire was administered to 170 inpatients. Charlson scores were abstracted from these patients' medical records. We assessed test-retest reliability of the questionnaire and the Charlson index, the correlation between the questionnaire and the Charlson index, and correlations between each comorbidity measure and indicators of health resource utilization including medication use, hospitalizations in the past year, and hospital charges. Test-retest reliability, assessed with the intraclass correlation coefficient, was 0.91 for the questionnaire and 0.92 for the chart-based Charlson index. The Spearman correlation between these two measures was 0.63. The correlation between comorbidity measures was weaker in less educated patients. Correlations with indicators of resource utilization were similar for the two comorbidity instruments. The authors found that a questionnaire version of the Charlson index is reproducible, valid, and offers practical advantages over medical record-based assessments.