OBJECTIVE : Nonsteroidal antiinflammatory drugs(NSAIDs) are widely used in the treatment of a number of arthritic conditions or lumbago, but gastrointestinal lesions have often limited their clinical utilization. Zaltoprofen is a new propionic acid derivative provided with marked antiiflammatory, antiarthritic, analgesic and antipyretic activities in previous Japanese clinical trials. A comparative, randomized, double blind clinical trial was designed to assess the efficacy, tolerability and safety of zaltaprofen(ZP) compared to ketoprofen(KP) in the treatment of lumbago. MATERIALS AND METHODS : A total of 191 adult patients (66 males and 125 females) suffering from lumbago participated in this double-blind, multicenter trial of 25-weeks duration. Using the randomization table, 96 patients(ZP group) were treated with 3 tablets daily of Zaltoprofen 80mg and placebo for 2 weeks; the other 95 patients(KP group) took 3 tablets daily of Ketoprofen 50mg and placebo for 2 weeks. The therapeutic efficacy was assessed using parameters, measured in baseline conditions and after 7, 14 days of treatment : 1) Subjective pain scale measured using visual analogue scale 2) Objective range of motion (Flexion, extension, abduction and rotation). The safety was assessed by : 1) Appearance during the study of subjective and objective side effects 2) Laboratory tests performed before study commencement and at the end of the therapy. The parametric and non-parametric data were analysed statistically using t-test, paired t-test, repeated measures of ANOVA and Chi-square test. RESULTS : Therapy with ZP or KP produced a marked improvement in subjective pain scale and objective range of motion in both groups of patients (p<0.0001 and p<0.05). But No statistically significant differences were observed between the two groups. 20 of 96 patients (20.83%) treated with ZP and 37 of 95 patients (38.95% ) treated with KP had side effects. 20 patients treated with ZP and 34 patients treated with KP had the gastrointestinal complaints. But the side effect was short-lasting and was disappeared a few days after discontinuation of therapy. The laboratory tests in both groups showed no statistically significant variations. CONCLUSION : Zaltoprofen provides an excellent therapeutic weapon for the treatment of lumbago. It possesses a marked analgesic activity and induces a satisfactory improvement of the range of motion and is generally equivalent to that achieved with ketoprofen. On the other hand, the frequency of side effects in zaltoprofen group is lower than that of ketoprofen group.