The efficacy and safety of carbocysteine lysine salt monohydrate (SCMC-Lys) in the prevention of acute exacerbations associated with chronic obstructive bronchitis were evaluated in a multicenter double-blind, placebo-controlled, parallel group trial in 662 outpatients. After a 1-month run-in period, patients were randomized to three groups and assigned to receive one of the following oral treatments: continuous SCMC-Lys 2.7 g once daily, intermittent SCMC-Lys at the same dosage (1-week courses alternating with 1-week intervals on placebo) or placebo. Each treatment lasted for 6 months and spanned the cooler months of the year. Evaluation was based on a daily recording of relevant clinical symptoms and signs and subsequent evaluation of the collected data by three blinded independent physicians. A total of 146 patients (23%) failed to complete the 6-month treatment (mostly due to difficulties in complying with protocol requirements), without clear-cut differences in the dropout rate in the three groups. An intention-to-treat analysis revealed that the incidence of patients without exacerbations in the group assigned to continuous SCMC-Lys treatment was significantly higher than in the placebo-treated group (p < 0.001). The total number of patients with at least one exacerbation was 66 (29.6%) in the group treated with continuous SCMC-Lys compared with 100 (45.9%) with placebo. In the former group the time between initiation of treatment and first exacerbation was significantly prolonged. The average number of days with acute respiratory illness was significantly decreased in the group receiving continuous SCMC-Lys compared with the group receiving placebo, and this was associated with a significant reduction in the antibiotic consumption during the trial period. In patients assigned to intermittent treatment, results of the assessed endpoints did not differ significantly from those observed in the placebo group. No serious adverse effects were reported. It is concluded that continuous administration of SCMC-Lys throughout the winter season is effective in preventing acute exacerbations in patients with chronic obstructive bronchitis and it is well tolerated.