Studies on the efficacy of antimicrobial agents against actinomycosis in vivo have been limited apart from those involving penicillin. A prospective ministudy on the efficacy of imipenem-cilastatin in the treatment of pulmonary actinomycosis was performed based on preliminary encouraging in vitro and in vivo data. Eight patients were diagnosed as having pulmonary actinomycosis using fibreoptic bronchoscopy (7) and percutaneous transthoracic needle biopsy (1) in the authors' unit between 1994 and 1996. Each patient received a 4-week course of imipenem-cilastatin that comprised 2 weeks of intravenously administered drug (500 mg at 8-hourly intervals) and 2 weeks of intramuscularly administered drug (500 mg at 12-hourly intervals). Seven patients showed a very good clinical and radiographic response as well as bronchoscopically-documented treatment success. Treatment failed in one patient. Amongst the former group, one patient was lost to follow-up, another relapsed 3 months after treatment cessation and the rest remained relapse-free when followed-up for 18-44 months (mean 30.2 months). Furthermore, all patients showed good clinical tolerance and no abnormal treatment-related laboratory findings. The favourable outcome for most patients in this mini-study suggest that a 4-week parenteral course of imipenem-cilastatin is an efficacious treatment for pulmonary actinomycosis. This antimicrobial regimen might be a promising alternative to the time-honoured long-course treatment with intravenous and oral penicillin.