Two groups of 20 patients each, with mild to moderate acute low back pain with associated muscle spasm of ten days' duration or less, were treated with a combination of cyclobenzaprine and naproxen or naproxen alone in a randomized, 14-day open-label trial. Cyclobenzaprine was added to the naproxen regimen as an adjunct to rest and physical therapy for relief of muscle spasm associated with acute, painful, musculoskeletal conditions. The clinical characteristics of each study group, including the number of worker's compensation patients, were comparable. Combination therapy was associated with less objective muscle spasm and tenderness and greater motion of the lumbosacral spine (P less than 0.05). There were trends toward faster resolution of functional deficits and pain with combined therapy. Combination therapy was associated with more side effects, due primarily to drowsiness from the cyclobenzaprine. The results of this study demonstrated that patients with muscle spasm associated with acute low back strain benefited from the use of combination therapy consisting of a nonsteroidal anti-inflammatory agent (naproxen) and a muscle relaxant (cyclobenzaprine).