This study evaluated changes in range of motion (ROM) in competitive male and female junior tennis players participating in a specific conditioning program over a two-year period. Subjects were also compared to an age and sex-matched control group of players not performing the exercises. Fifty-one tennis players, 29 male (mean age 13.6) and 22 female (mean age 13.2) entered the study. Baseline ROM measurements were performed on all players upon entering the study. The conditioning program employed standardised stretch-hold-relax flexibility exercises for all the areas tested. These exercises were taught to the players and each athlete was given a video of the exercises. Compliance with the program was assessed by exercise logs, the parents, and the athletes. High compliers were compared to low compliers. Mean changes in the combined ROM measurements over time indicate improvement in flexibility in most anatomical areas after one year of participation, with small improvements in the second year. With the exception of forearm supination, there were no differences between high compliers and low compliers. Compared to controls, the experimental group showed significant improvement in sit and reach, dominant and nondominant shoulder internal rotation, dominant and nondominant shoulder external rotation, nondominant gastrocnemius, dominant iliotibial band, dominant and nondominant hip internal rotation, dominant and nondominant hip external rotation, dominant and nondominant forearm pronation, and dominant wrist flexion. The areas of greatest significant change were in the shoulder and back, which correspond to the areas of most clinical risk of injury. In conclusion, joint ROM can be positively influenced with a specific conditioning program.