Strongyloides stercoralis is an intestinal nematode that is able to infect the host tissue and persist for many years through autoinfection, and it causes life-threatening hyperinfection in immunocompromised hosts. We report here on two cases of strongyloidiasis that were diagnosed by colonoscopic biopsy. One case was a 73-year-old woman who was hospitalized with complaints of melena. She was being treated with corticosteroid due to her asthma and rheumatoid arthritis. The other case was a 63-year-old man who suffered with abdominal discomfort and severe loss of body weight (18 kg) for 2 months. In both cases, colonoscopic examination revealed polyps and petechiae at the entire colon. Microscopically, a small illdefined granuloma with a longitudinally sectioned parasite was seen on the colonoscopic biopsy. Endoscopic examination was done after suspecting parasitic infestation. The gastric and duodenal mucosa showed numerous cross sections of adult worms, eggs and larvae that were developing in crypts. Even if such a patient is in an asymptomatic state, this illness must be treated due to the potential for fatal autoinfection.