In brief: The incidence of shoulder pain was studied in 168 male and female swimmers, aged 12 to 23, representing four universities and two clubs. A simple test of shoulder flexibility was administered in October, and team- wide incidence of swimmer's shoulder for the coming winter season was predicted. The predictions proved to be about 93% accurate, clearly showing a strong correlation between lack of flexibility and incidence of swimmer's shoulder. In males, increased incidence of shoulder pain was also related to increased intensity and duration of weight training. The incidence of shoulder pain was not directly dependent on stroke, distance, or sex, with one exception: At a given level of flexibility, butterfly swimmers showed a higher incidence of shoulder pain than other swimmers.