The release of tissue-type plasminogen activator (t-PA) after venous occlusion was tested in 290 patients with venous thrombosis, at earliest three months after the last thrombo-embolic episode. In 37 patients (12.8%) the t-PA-activity was decreased. In 14 patients (4.8%) the defect was confirmed by control. A reduced release of t-PA was found in 4 patients, while 10 out of 14 patients had comparatively higher levels of plasminogen-activator-inhibitor (PAI-I).