Proteases are important for neoplastic invasion but a specific role for the plasminogen activator system in the progression of colorectal epithelial dysplasia to adenomatous lesions remains unclear. Consecutive tissue cryosections of 51 adenomas, 49 distant mucosa samples and five mucosa samples from control subjects were histopathologically analysed for dysplasia grade and tissue type, urokinase plasminogen activator levels and plasminogen activator inhibitor type 1 (PAI-1) using immunosorbent methods. Plasminogen activation and urokinase-mediated proteolytic activity levels were assessed using in situ zymography. Plasminogen activation and tissue-type activator levels were lower in adenomas than in mucosae (P < 0.001). PAI-1 concentration and urokinase levels were higher in adenomas than in mucosae (P < 0.001 and P < 0.001 respectively). In adenomas, urokinase concentration increased in parallel with PAI-1, but only the urokinase levels correlated with the dysplasia grade (P < 0.01). Thus, the alterations in plasminogen activation correlated with epithelial cell dysplasia grading. In the mucosa to adenoma transition, a marked decrease in tissue-type plasminogen activator occurred. In adenomas, this decrease was accompanied by a concomitant increase in urokinase and PAI-1. The urokinase level only continued to rise in parallel with the dysplasia grade. Resulting protease-antiprotease imbalance in high-grade dysplasia may represent the phenotypic change associated with malignant transformation and invasive behaviour.