Previous studies identified serine, cysteine and metalloproteases in normal aqueous humours (AH) and suggested that a balance between proteases and their inhibitors may play a role in the modulation of the AH outflow. We aimed to determine whether secretory leukocyte protease inhibitor (SLPI), a serine protease inhibitor, is present in AH of patients with cataract and other eye pathologies. AH was collected from 117 cataract patients of which 55 were diagnosed with more when one eye disease: cataract only (n=62), pseudoexfoliation (PEX) (n=26), glaucoma (n=6), diabetes retinopathy (n=4), iritis-uveitis (n=4) and macular degeneration (n=28). The total protein in AH was determined by a Bradford assay and SLPI was analyzed by Western blot and ELISA methods. The average concentration of total protein and SLPI in AH samples was 160+/-15 microg/ml (n=117, +/-SEM) and 500+/-94 pg/ml (n=105), respectively. The cataract patients with additional eye disease(s) showed higher protein levels (201+/-35 microg/ml) than cataract (controls) (128+/-31 microg/ml), P<0.01. It is noteworthy that no correlation was found between SLPI and the total protein concentrations in AH, but SLPI was positively correlated with age (r=0.2, P<0.05). No statistical difference in SLPI levels was found between controls (cataract) and other pathologies, while patients with iritis/uveitis had higher SLPI levels compared to those with diabetes (P<0.05). We show here for the first time that SLPI is present in AH and may play a role as well as serve as a marker in pathological states.