Forty-three patients suffering from nasal polyposis underwent intranasal treatment with increasing doses of lysine acetylsalicylate (LAS) corresponding to 20, 200, and 2000 micrograms of aspirin (ASA), until a maximal dose of 2000 micrograms weekly was reached. The patients were divided in two groups: a group of 28 patients with ASA intolerance, including 20 with ASA triad, and a group of 15 patients without ASA intolerance. The local treatment was usually started 1 month after polypectomy and was well tolerated without side effects. A control group included 191 subjects with nasal polyposis, 130 of whom had ASA intolerance. After polypectomy the controls received no further medical treatment. Patients were examined every 3 months and radiographs of the paranasal sinuses were obtained every 6 months. After 24 months 34 of 43 patients (79.1%) treated with topical LAS had suffered no relapse of polyposis. Only 45 of 191 control patients (23.6%) failed to relapse after 24 months (P less than .0001). Nine of 28 (32.1%) ASA-intolerant patients treated with LAS and 105 of the 130 (80.77%) control subjects relapsed (P less than .0001). None of the 15 ASA-tolerant patients treated with LAS relapsed, but 41 of the 61 (67.21%) nontreated control subjects relapsed (P less than .00001). These data indicate topical LAS is effective in preventing recurrence of nasal polyps after polypectomy.