The aim of this study was to determine the presence of angiostatin in ascitic and pleural effusions from cancer patients, as well as of metalloproteinases (MMPs) and urokinase-type plasminogen activator (uPA), both involved in angiostatin generation in in vitro models. Ascitic fluids, pleural exudates, and sera from 21 cancer patients were analyzed for the presence of angiostatin by western blot, whereas gelatinases MMP-2 and MMP-9, and uPA were evaluated by zymography. Our study revealed elevated levels of angiostatin in effusions of cancer patients, contrasting with mostly intermediate levels in less than half of their sera, and undetectable levels in normal sera. Despite the observation of enhanced levels of HMW-uPA and MMP-2 in malignant effusions from cancer patients, their analysis in individual samples showed no association between angiostatin presence and the enzymes, suggesting that the latter would not play an unimportant role, if any, in in vivo generation of angiostatin.