A double-blind study was carried out to determine the effectiveness of a muscle relaxant, carisoprodol, in the treatment of the low back syndrome, and to test whether this drug would produce any greater effect than an active sedative control. Forty-eight Mexican migrant farm laborers with acute lumbar strain and spasm were given either carisoprodol 350 mg, butabarbital 15 mg, or placebo, four times daily for four days, and then were rated on pertinent symptoms, the degree of limitation of motion (by an objective finger-to-floor test), and overall improvement. In the 43 patients who could be statistically analyzed, carisoprodol was shown to be significantly more effective than butabarbital or placebo in producing improvement of all factors rated. This result suggests that the effects of carisoprodol are not due solely to sedative action, but are also related to its muscle relaxant activity.