In recent decades, evidence has been accumulating showing the important role of urokinase-type plasminogen activator (uPA) in growth, invasion, and metastasis of malignant tumours. The evidence comes from results with animal tumour models and from the observation that a high level of uPA in human tumours is associated with a poor patient prognosis. It therefore initially came as a surprise that a high tumour level of the uPA inhibitor plasminogen activator inhibitor-I (PAI-I) is also associated with a poor prognosis, the PAI-I level in fact being one of the most informative biochemical prognostic markers. We review here recent investigations into the possible tumour biological role of PAI-I, performed by animal tumour models, histological examination of human tumours, and new knowledge about the molecular interactions of PAI-I possibly underlying its tumour biological functions. The exact tumour biological functions of PAI-I remain uncertain but PAI-I seems to be multifunctional as PAI-I is expressed by multiple cell types and has multiple molecular interactions. The potential utilisation of PAI-I as a target for anti-cancer therapy depends on further mapping of these functions.