Clarithromycin is a new semisynthetic macrolide, Erythromycin A derivative, which is bactericidal for the most of growing aerobic and anaerobic Gram-positive and Gram-negative organisms. The drug has also a potent activity against the pathogens surviving into intracellular medium and, just like all other macrolides, achieves its effect by inhibiting protein synthesis. Our study investigated the efficacy and tolerability of Clarithromycin in 11 ambulatory patients (8 m, 3f) suffering from bacterial relapses of chronic bronchitis. The macrolide was administered at the oral dosage of 250 mg twice daily for a period ranging from 7 to 11 days, only after microbiological evaluation of sputum. There was a withdrawal because one of the three bacteria isolated from sputum sample was resistant to Clarithromycin, 7 patients were clinically cured, 3 showed only clinical improvement. In all the ten patients there was eradication of causative agents. No adverse events or changes in biochemical and haematological tests were observed.