This study evaluated the reliability of a new medication appropriateness index. Using the index, independent assessments were made of chronic medications taken by 10 ambulatory, elderly male patients by a clinical pharmacist and an internist-geriatrician. Their overall inter-rater agreement for medication appropriateness (ppos) was 0.88, and for medication inappropriateness (pneg) was 0.95; the overall kappa was 0.83. Their intra-rater agreement for ppos was 0.94 overall, for pneg was 0.98 overall while the overall kappa was 0.92. The chronic medications taken by 10 different ambulatory elderly male patients were independently evaluated by two different clinical pharmacists. Their overall inter-rater agreement for ppos was 0.76, and for pneg was 0.93, while the overall kappa was 0.59. This new index provides a reliable method to assess drug therapy appropriateness. Its use may be applicable as a quality of care outcome measure in health services research and in institutional quality assurance programs.