OBJECTIVE: This article describes the development, administration, and reliability of the Cornell Services Index (CSI), a new instrument that measures health service use. The CSI was developed to create a standardized measure of the quantity and characteristics (for example, site and provider) of services used by adults. Descriptive data are provided to illustrate the application of the CSI in a community sample of adults who were newly admitted to outpatient mental health clinics. These data provide information about the pathways to care.

METHODS: The interrater and test-retest reliability of the CSI were evaluated by using a sample of 40 adults who were seeking mental health treatment. Descriptive data on service use in a sample of 1,279 adults seeking care in outpatient mental health clinics was provided to demonstrate the application of the CSI.

RESULTS: The CSI is a portable, easy to use, and brief assessment of service use. It has good interrater and test-retest reliability among adults without cognitive impairment. In the three months before seeking care, 31 percent of the adults interviewed had made a mental health visit, 36 percent had been hospitalized, and more than half (59 percent) had made a medical visit. Twenty-three percent of adults had sought care from a hospital's emergency department.

CONCLUSIONS: The CSI is a reliable method to assess health service use for adults. The measure can extend assessment of use beyond the traditional mental health service use questions and provide a snapshot of service use patterns across types, providers, and sites of service among adults who seek mental health care.