Cyclospora cayetanensis has been observed in the feces of persons with prolonged diarrhea. A description of the symptoms and histopathologic findings for patients with cyclosporiasis is presented. The intracellular life-cycle stages of these parasites in the enterocytes of patients will also be described. Seventeen Peruvian patients positive for Cyclospora organisms were surveyed and underwent endoscopy, and their symptoms were recorded. Patients presented with gastrointestinal symptoms, including diarrhea, flatulence, weight loss, abdominal discomfort, and nausea. Jejunal biopsies showed an altered mucosal architecture with shortening and widening of the intestinal villi due to diffuse edema and infiltration by a mixed inflammatory cell infiltrate. There was reactive hyperemia with vascular dilatation and congestion of villous capillaries. Parasitophorous vacuoles contained sexual and asexual forms. Type I and II meronts, with 8-12 and 4 fully differentiated merozoites, respectively, were found at the luminal end of epithelial cells. These findings demonstrate the complete developmental cycle associated with host changes due to Cyclospora organisms.