[영문]Purpose. We report the validation of an instrument to measure mistrust of health care organizations and examine the relationship between mistrust and health care service underutilization.Methods. We conducted a telephone survey of a random sample of households in Baltimore City, MD. We surveyed 401 persons and followed up with 327 persons (81.5 percent) 3 weeks after the baseline interview. We conducted tests of the validity and reliability of the Medical Mistrust Index (MMI) and then conducted multivariate modeling to examine the relationship between mistrust and five measures of underutilization of health services.Results. Using principle components analysis, we reduced the 17-item MMI to 7 items with a single dimension. Test?retest reliability was moderately strong, ranging from Pearson correlation of 0.346?0.697. In multivariate modeling, the MMI was predictive of four of five measures of underutilization of health services: failure to take medical advice (b=1.56, pqqqlt;.01), failure to keep a follow-up appointment (b=1.11, p=.01), postponing receiving needed care (b=0.939, p=.01), and failure to fill a prescription (b=1.48, p=.002). MMI was not significantly associated with failure to get needed medical care (b=0.815, p=.06).Conclusions. The MMI is a robust predictor of underutilization of health services. Greater attention should be devoted to building greater trust among patients.