One hundred and forty outpatients, males and females aged 14 to 70 years, with acute febrile bronchitis, afebrile chronic bronchitis, or flares of chronic bronchitis were treated for seven days with one soft gelatin capsule (500 mg) of guaimesal thrice daily or matching placebo. No other potentially interfering treatment (antibiotic, antipyretic, or anti-inflammatory) was administered concomitantly. Before and after treatment, vital capacity, residual volume, and maximum expiratory volume per second were measured and recorded. Both before treatment and after 2, 4, and 7 days of treatment, body temperature was measured and cough frequency and intensity as well as expectorate amount and fluidity were scored and recorded along with the total symptom score. At the end of treatment, the overall clinical efficacy was rated and recorded. Potential side effects were monitored by questioning the patients; general safety (laboratory tests) was not monitored. All data were analyzed according to multivariate, parametric, and nonparametric statistical methods and stratified by treatment, sex, age, and diagnosis. For each monitored symptom and sign, guaimesal exhibited a favorable effect which was significantly greater in extent and significantly faster in recovery rate than that associated with spontaneous recovery. Within two days from the start of therapy, fever, cough frequency and intensity, and expectorate fluidity had significantly improved. By the fourth day the expectorate amount had significantly decreased. Further improvement was noted at subsequent times of observation. After a week, according to the spirometric parameters, the respiratory function was restored to a considerable extent. Overall, 75.7% of the patients receiving guaimesal experienced complete or almost complete recovery within the period of observation. With placebo, occasional improvement was seen for cough intensity and for expectorate amount, and this improvement was significantly less in extent and significantly slower than that recorded with guaimesal. Overall, 8.6% of placebo-treated patients experienced a clinically detectable degree of recovery. The subjective tolerance of guaimesal was comparable to that of placebo in terms of both the frequency and the nature of the side effects. Mild, transient gastric effects were reported by 11.4% of patients receiving guaimesal and by 8.6% of patients receiving placebo. These effects did not require a variation of dose or specific treatment. The clinical-therapeutic efficacy and tolerance and guaimesal were similar in all groups regardless of age, sex, or disorder treated, even though conditions on entry were significantly different as to specific disorders. Guaimesal is appropriate for the treatment of acute, subacute, and chronic obstructive disorders of the airways. It exerts a number of coordinate, useful actions-especially mucolytic-expectorant, antipyretic, and anti-inflammatory actions-with negligible risk of minor side effects. Moreover, as a monocompound it relieves the physician and the patient of the trouble and risk associated with alternative fixed-combination or multiple-drug treatments.