Parasitic infestation of the intestinal tract was investigated in a prospective study of 177 patients from southern Europe and Turkey. In a group of patients (n = 127) who attended the Medical Outpatients Department because of abdominal pain, pathogenic parasites were isolated from a single stool sample in 69 (54%). 19 patients had 2 or more parasites. 22 out of 25 patients reviewed after treatment were free of parasites, and 17 were symptom-free. Abdominal pain persisted unchanged in 8 patients. In a second group of patients (n = 50) with extraabdominal symptoms, 11 (22%) had pathogenic parasites in stool. Multiple infestation did not occur in this group. The results show that in this population group stool examination for parasites should be carried out routinely in the investigation of abdominal pain. The high prevalence rate possibly justifies a search for parasites even where there are no intestinal symptoms.