OBJECTIVE: To compare the changes in hamstring length resulting from modified proprioceptive neuromuscular facilitation flexibility training in combination with cold-water immersion, hot-water immersion, and stretching alone. DESIGN AND SETTING: Training-only subjects stood motionless for 10 minutes, while subjects in the cold group stood in a cold-water bath (8 degrees +/- 1 degrees C) immersed up to the gluteal fold for 10 minutes, and subjects in the hot group stood in a hot-water bath (44 degrees +/- 1 degrees C) immersed up to the gluteal fold for 10 minutes. All subjects exercised only the right lower limb using a modified proprioceptive neuromuscular facilitation flexibility protocol, consisting of 1 set of 4 repetitions. This procedure was followed for 5 consecutive days. SUBJECTS: Forty-five uninjured subjects (21 women, 24 men; age range, 18-25 years) were randomly assigned to the cold, hot, or stretching-alone group. MEASUREMENTS: Subjects were measured for maximum active hip flexion on the first and fifth days. RESULTS: Group results were assessed using a 2 x 3 analysis of variance, comparing changes in hamstring length from pretest to posttest. All 3 groups had significant improvements in hamstring length (pretest to posttest) (P <.05). However, no significant differences occurred among groups. CONCLUSIONS: No advantage was apparent in using complete hot or cold immersion to increase hamstring length in healthy subjects.