Although hand-held dynamometry is considered an objective method of measuring strength, the reliability of the procedure can be compromised by inadequate tester strength and insufficient stabilization of the dynamometer and subject. The purpose of this study was to investigate the test-retest reliability of a hand-held dynamometer with the use of a portable stabilization device while testing the shoulder internal and external rotator musculature. The isometric strength of the shoulder rotator musculature was tested twice in 30 asymptomatic adult volunteers (15 male and 15 female) between 18 and 63 years of age by using an intrasession design. Consistency of the testing protocol was maintained through the use of an arm stabilization apparatus, which fixed the arm in 30 degrees of the scapular plane and a portable dynamometer stabilization device. Intraclass correlation coefficients (ICC's) were high, ranging from ICC (3,1) = 0.971-0.972 for the test-retest trials of internal and external rotation. There was no significant difference between sessions one and two for maximum internal rotation (p = 0.431) and maximum external rotation strength (p = 0.780). The results indicate that the testing protocol with stabilization device is a reliable method for measuring strength of the internal and external rotator shoulder musculature.