The clinical effect of N-acetylcysteine (NAC) controlled-release tablets, 300 mg b.i.d., and placebo, in chronic bronchitis was investigated. The study was performed as a double-blind six month comparison between active drug and placebo in two parallel groups, with statistical evaluation after four and six months. The patients were chosen from nine centres. One hundred and sixteen out-patients were included and ninety one of them completed the six month study. The acetylcysteine-treated group had a significantly reduced number of sick-leave days caused by exacerbations of chronic bronchitis after the four winter months December-March compared with the control group (NAC 173, placebo 456). The number of exacerbation days was also very much reduced, however, not significantly (NAC 204, placebo 399). At the end of the six month trial, including also two spring months, the absolute numbers of sick-leave days and exacerbation days were still fewer in the acetylcysteine-treated group, (NAC 260, placebo 739) and (NAC 378, placebo 557) respectively. This study demonstrates a significant reduction in sick-leave days after four months of NAC-treatment. A constant tendency to reduction in the number of exacerbations and exacerbation days was also registered after four and six months. The differences in these parameters were, however, not statistically significant. This was probably due to the small number of patients participating.