Objective: To assess patient satisfaction and a health plan's return on investment associated with a telephone-based triage service. Study Design: A pre-post study design, with medical claims data, to assess changes in medical service utilization and health plan expenditures associated with members' use of the triage service. Patients and Methods: This study is based on data on 60,000 members of a health plan. A telephone survey was conducted to assess member satisfaction and outcomes with the triage service. The plan's medical claims and encounter data were used to calculate medical utilization rates and plan expenditures for those services. The health plan's return-on-investment was evaluated using a pre/post study design to assess changes in medical service utilization between the baseline (December 1995 through November 1996) and program (December 1996 through November 1997) periods. Results: The average nurse response time to a call was just less than 50 seconds, which indicates the service provided ready access to medical advice 24 hours per day, 7 days per week. More than 90% of users were satisfied, and utilization of hospital emergency department (ED) and physician office services decreased significantly after the service was implemented. The changes in medical service utilization resulted in reductions in health plan expenditures that exceeded the plan's costs of providing the service. The plan's estimated return for every dollar invested in the nurse triage service was approximately $1.70. Conclusions: The telephone-based nurse triage service appears to be a cost-effective intervention that improves access to medical advice, thereby encouraging appropriate use of medical services. The service is associated with reductions in utilization of hospital ED and physician office services and with high levers of member satisfaction.