Seventy out-patients with acute back pain participated in a double-blind comparative trial of the clinical efficacy and tolerance of orally administered meptazinol and diflunisal. Half of the patients received 200 mg meptazinol or 250 mg diflunisal 4-times daily for up to 3 weeks, depending on the duration of pain. Patients were examined 4 times at 1-week intervals for their capability to do daily tasks, for their capacity for forward bending, thoraco-lumbar torsion, straight leg raising, static hip flexion and sit-ups, and for subjective assessment of pain. Side-effects were recorded on a questionnaire. Both treatments produced marked improvement in most of the parameters assessed, often within the first week and, overall, the results were similar with the two drugs. Few side-effects were reported and those that were recorded were slight and similar in incidence apart from nausea in 5 meptazinol-treated patients and smarting and burning on urination in 2 patients receiving diflunisal.