background. a great deal of controversy still exists about using antibiotic therapy in association with the tested efficacy of closed chest drainage.the authors studied 167 patients with isolated thoracic trauma to assess the importance of antibiotic therapy to the hospitalization period of post closed chest drainage cases. methods: a longitudinal prospective study of cohorts was conducted by randomly selecting two groups of patients. the control group consisted of 104 patients who were not given antibiotics and 63 patients in the experimental group were given cephalothin sodium during the post operative period. (500 mg iv - 6/6 hrs). results : the patients had suffered the following kinds of injuries: closed trauma -12 cases (7.2%); knife injuries - 98 cases (58.7%); firearm injuries - 41 cases (24.6%); lesions caused by other agents - 16 cases (9.6%). the results showed that the average period of hospitalization for the control group was 5.7±3.2 days and for the antibiotic group it was 5.7±2.9 days. conclusion: the results indicated that hospitalization period for the patients in the control group, who were not given antibiotics and who developed some kind of pleuropulmonary complication, was 3.2 days more than the others.