METHODS A prospective randomized clinical trial compared the effects of flexion and extension back exercises and postures among soldiers with acute low back pain. OBJECTIVE To compare the immediate effects of back exercise on functional status, spinal mobility, straight leg raising, pain severity, and treatment satisfaction, and to determine whether spinal exercise during the acute stage of low back pain reduces recurrent episodes of low back pain. BACKGROUND Conflicting reports exist concerning the efficacy of spinal flexion and extension exercises in patients with low back pain of varying duration. Poor study design and lack of functional outcomes characterize many of these studies. METHODS One-hundred-forty-nine subjects with acute low back pain received flexion exercise and posture (n = 57), extension exercise and posture (n = 62), or no exercise or posture (n = 30) for 8 weeks. Outcomes were assessed 1, 2, 4 and 8 weeks after treatment onset. A questionnaire assessed the recurrence of low back pain 6-12 months after study entry. RESULTS Flexion and extension exercise groups did not differ in any outcome over 8 weeks. After 1 week, both exercise groups had reduced disability scores, a higher proportion returning to work, and fewer subjects with a positive straight-leg raise compared with the control group. There was no difference among groups regarding recurrence of low back pain after 6-12 months. CONCLUSIONS There was no difference for any outcomes between the flexion or extension exercise groups. However, either exercise was slightly more effective than no exercise when patients with acute low back pain were treated.