Intrapleural administration of fibrinolytics has been shown in small numbers of patients with complicated parapneumonic effusions (CPE) and pleural empyema to be effective and relatively safe. Although streptokinase (SK) is recommended as the fibrinolytic of choice, there are no comparative studies among fibrinolytics. We therefore compared the efficacy, safety, and the cost of treatment two of the most used thrombolytics, SK and urokinase (UK). Fifty consecutive patients with CPE or empyema were randomly allocated to receive either SK (25 patients) or UK, in a double-blind fashion. All patients had inadequate drainage through chest tube (< 70 ml/24 h). Both drugs were diluted in 100 ml normal saline and were infused intrapleurally through the chest tube in a daily dose of 250,000 IU of SK or 100,000 IU of UK. The chest tube was clamped for 3 h after instillation. Response was assessed by clinical outcome, fluid drainage, chest radiography, pleural ultrasound, and/or computed tomography. Clinical and radiologic improvement was noted in all but two patients in each group, who required surgical intervention. The mean volume drained during the first 24 h after instillation was significantly increased; 380 +/- 99 ml for the SK group (p < 0.001) and 420.8 +/- 110 ml for the UK group (p < 0.001). The total volume (mean +/- SD) of fluid drained after treatment was 1,596 +/- 68 ml for the SK group, and 1,510 +/- 55 ml for the UK group (p > 0.05). The SK instillations (mean +/- SD) were 6 +/- 2.16 (range, 3 to 10) and those of UK 5.92 +/- 2.05 (range, 3 to 8). High fever as adverse reaction to SK was observed in two patients. The total cost of the drug in the UK group was two times higher than that of SK ($180 +/- 47 for SK and $320 +/- 123 for UK). The mean total hospital stay after beginning fibrinolytic therapy was 11.28 +/- 2.44 d (range, 7 to 15) for the SK group and 10.48 +/- 2.53 d (range, 6 to 18) for the UK group (p = 0.32). We conclude that intrapleural SK or UK is an effective adjunct in the management of parapneumonic effusions and may reduce the need for surgery. UK could be the thrombolytic of choice given the potentially dangerous allergic reactions to SK and relatively little higher cost of UK.