The purpose of this study was to evaluate for diagnostic purposes the isometric strength of the neck flexor and extensor muscles in 56 patients with neck and shoulder pain and to quantify the effect of a specific rehabilitation program. An isometric measurement protocol was performed at the beginning and at the end of a 3-week rehabilitation program that included physiotherapy, stretching, aerobic exercises, and circuit training to improve arm, shoulder, and neck muscle strength. In the beginning, mean neck flexor muscle isometric strength was 83N (+/- 48 SD) and extensor muscle strength 158N (+/- 76 SD), whereas after rehabilitation the strength measurements were 117N (+/- 43 SD) and 207N (+/- 84 SD) respectively. There was a significant increase in the ability to push forward and backward, which correlated with the lessening of neck pain and disability found at the end of the program (p < .05). Isometric strength measurement is a useful and practical method of objectively showing a functional improvement in response to rehabilitation.