This prospective, multicenter, randomized, "unblinded," controlled clinical trial was designed to determine if the intrapleural instillation of 1500 mg of tetracycline hydrochloride would be effective in diminishing the ipsilateral rate of recurrence for spontaneous pneumothorax. During the 4-year enrollment period, 113 patients were assigned to the tetracycline group; 116 patients were assigned to the control group. During the 5-year study period, the recurrence rate in the tetracycline group (25%) was significantly less than that in the control group (41%). Use of tetracycline seemed to reduce the recurrence rates for patients with either primary or secondary spontaneous pneumothorax and for patients with either an initial or a recurrent pneumothorax. We conclude that the intrapleural administration of tetracycline in patients with spontaneous pneumothorax significantly reduces the rate of ipsilateral recurrence but is associated with intense chest pain. Intrapleural tetracycline therapy is indicated for patients with a spontaneous pneumothorax who are hospitalized and are treated with tube thoracostomy.