The effects of two urokinase (UK) dose regimens on lysis time, lytic success, primary clinical success, and frequency of complications of peripheral thrombolysis were compared. Seventy-two intraarterial UK infusions were performed by means of standard catheter-directed infusion techniques in 63 patients with symptomatic peripheral arterial or bypass graft occlusions. Patients were prospectively randomized to high-dose (250,000 U/h for 4 hours and then 125,000 U/h) or low-dose (50,000 U/h) regimens. The mean time to complete lysis was 20.8, 26.0, 16.5, and 18.2 hours for the high-dose artery, low-dose artery, high-dose graft, and low-dose graft groups, respectively (P was not significant). Respective mean infusion durations were 27.1, 35.4, 22.2, and 25.3 hours. Clinical success was achieved in 65%-85% of cases. The frequency of complications was equivalent between groups, except for a higher frequency of minor bleeding complications in the high-dose group. The two urokinase dose regimens studied were equally effective in enabling peripheral thrombolysis.