The validity of the Index of Medical Underservice (IMU) was evaluated by examining the ability of the IMU to discriminate among levels of need for health services reported by rural consumers in response to a mail survey questionnaire. It was hypothesized that if the IMU is a valid indicator of medical underservice, then where the IMU value for an area was relatively low, respondents would report relatively less access and availability of health services, less utilization of health services, a lower health status level, and less satisfaction with available health services. The IMU was not found to be an effective discriminator among levels of need for health services as reported by the survey respondents. Further, the dichotomous designation of areas as medically underserved or not medically underserved according to IMU values was found to display even less discriminatory ability. A comparison of areas at opposite extremes of the range of IMU values accounted for only a small proportion of the variance in six criterion measures. The findings raise serious questions about the utility of the IMU for determining funding priorities for health services programs. It is recommended that use and interpretation of the IMU be made with caution and supplemented by additional data as much as possible.