Changes in canalicular permeability following the insertion of punctum plugs was studied. This occlusive method was used to treat dry eye syndrome in 43 patients over a period of 32 months. All had normal canalicular patency, confirmed on the day of implantation. One hundred and five large plugs were used (2.4 plugs per patient). Twenty seven (25.7%) impassable acquired canalicular stenoses were seen in 15 patients. The canalicular site of the 27 stenoses was the same: junction between the vertical and horizontal portions. Among the 27 stenoses: 4 plugs had disappeared at an unknown time, with follow-up probing taking place after a mean interval of 7.5 (months range: 1 to 15 months); 17 plugs had been expelled after being in place for an average of 5 months (range: 1 to 12 months; standard deviation 3.47), with follow-up examination taking place on average 12 months after insertion (range: 6 to 17 months; standard deviation 3.4); and 6 plugs were still in place after a mean of 7 months (range: 2 to 15 months; standard deviation 4.73) and the stenosis was discovered when they were removed. They were intact. These 6 Cases eliminate any possibility of intra-canalicular migration of the prosthesis--of all or part--of the prosthesis. Stenoses appeared to develop rapidly: 45% of cases were discovered during the first 3 months. 16/27 (59%) of stenoses were investigated a second time, 6 months later on average (range: 2 to 12 months): these 16 canaliculi all remained impassable.