Surgical thoracostomy tube placement and radiologically guided catheter drainage are standard therapy for loculated pleural fluid collections. Treatment may fail if the catheter is not placed optimally within the loculation or if the fluid is hemorrhagic or fibrinous. We studied the value of transcatheter urokinase instillation in facilitating drainage of hemorrhagic or fibrinous nonhemorrhagic loculated pleural collections in 11 patients with 13 loculated pleural collections. Eight of the fluid collections were hemorrhagic, five were nonhemorrhagic. Five patients had had a thoracostomy tube placed surgically and all had had radiologically guided placement of single lumen drainage catheters managed with suction, saline irrigation, and mechanical guidewire manipulation. This therapy had failed to drain the loculations completely over an average of 10 days (range, 1-22 days). Urokinase (1000 units/ml) was instilled into the drainage catheters in 80- to 150-ml aliquots. After 1-2 hr, suction was reinstituted and the procedure was repeated. Twelve (92%) of the 13 collections were drained completely after an average of 4.3 instillations (range, three to eight instillations). Successful urokinase therapy required an average of 28 hr (range, 8-75 hr). In one case, therapy was discontinued after partial resolution for unrelated clinical reasons. There were no complications. These results suggest that transcatheter intracavitary urokinase therapy is a safe and effective method to facilitate drainage of loculated hemorrhagic or fibrinous nonhemorrhagic pleural fluid collections.