The purposes of this study were to develop a protocol to examine the reliability of manual muscle testing in a clinical setting and to use that protocol to assess the interrater reliability of manually testing the strength of the middle trapezius and gluteus medius muscles. One hundred ten patients with various diagnoses participated as subjects, and 11 physical therapists participated as examiners in this study. The results showed that interrater reliability for right and left middle trapezius and gluteus medius muscles was low. The percentage of therapists obtaining a rating of the same grade or within one third of a grade ranged from 50% to 60% for the four muscles. This study indicates that using manual muscle testing to make accurate clinical assessments of patient status is of questionable value.