In 1986 and 1987 the stool samples of 206 patients of the Medical Outpatient Clinic Basel were examined prospectively for bacteria, protozoa and worms. Clinical data of the patients were recorded by questionnaire. The patient group comprised 63 immunocompetent patients as well as 23 HIV-infected patients, all with symptoms of acute enteritis. The control group consisted of 120 healthy persons. Pathogenic organisms were found in the stools of 17.5% of the immunocompetent patients with enteritis. The most common germs were Campylobacter jejuni, Giardia lamblia and Salmonella enteritidis. Isospora belli, found in 2 cases (8.7%), was the only pathogen found in HIV-infected patients. The most frequent pathogen found in 7.5% of the control group was Giardia lamblia. The facultative pathogenic protozoon Blastocystis hominis was found in 16.7-19.0% of the 3 groups. There was no correlation between clinical symptoms and the results of stool examinations. Stool examinations in the immunocompetent patients corresponded to the known distribution of pathogenic germs in Switzerland. The homogeneous distribution of Blastocystis hominis in the 3 groups examined proves the high rate of infection. There is no evidence of a significant correlation between clinical symptoms of enteritis and infection with Blastocystis hominis in immunocompetent patients.