Hospitalized and domiciliary patients were studied to determine the incidence of the endemic nematode Strongyloides stercoralls in stool samples. Strongyloides was found in 14 (6.1%) of 229 hospitalized patients and in nine (2.6%) of 346 domiciliary patients. Clinical symptoms, laboratory data, and underlying diseases were compared for stool-positive and stool-negative patients. Infected patients were more likely to complain of abdominal bloating. They had a higher incidence of eosinophilia and guaiac-positive stools. They were more likely to have been treated with corticosteroids, cimetidine, and antacids. Efficacy of treatment with thiabendazole was studied in all stool-positive patients; a relapse rate of 15% was noted with standard thiabendazole therapy.