This multicenter, placebo-controlled, double-blind, randomized parallel-group trial was conducted to investigate the efficacy and tolerability of myrtol standardized (MYS, Gelomyrtol forte, 3 x 300 mg) in the long-term treatment of patients with chronic bronchitis during the winter. 246 patients received the investigational treatments (MYS: 122, placebo: 124) for at least 1 month; 215 subjects (110 under MYS and 105 under placebo) were evaluable in terms of efficacy (exacerbation rate, the need for antibiotics, symptom scores and general well-being) for the protocol-defined 6 months of treatment. Statistically significantly (p < 0.01) more patients remained without acute exacerbation in the myrtol standardized group (72%) compared to the placebo group (53%). In the placebo group, there was an evident peak in the incidence of exacerbations during the third month of treatment, which was not observed in the active treatment group. In the MYS group, 51.6% of the patients with an acute exacerbation required antibiotics vs. 61.2% under placebo. 62.5% of the patients treated with antibiotics in the MYS group required them for < or = 7 days, whereas 76.7% of the patients in the placebo group treated with antibiotics for exacerbation needed antibiotics for > 7 days. Well-being (assessed in terms of general health and health impairment by cough and expectoration) was significantly better under treatment with MYS. The overall therapeutic efficacy evaluation scored higher for MYS. Therefore, it is concluded that long-term treatment with MYS is equally well tolerated as placebo but is clearly superior in efficacy in terms of protecting against acute exacerbations in patients with chronic bronchitis: it reduces the frequency and intensity of acute exacerbations, the need of antibiotics for them and the health impairment by cough and expectoration.