The authors have performed nine rectoscopies in 14 cases of pseudomembranous colitis. A rectoscopic appearance enables eight times out of nine to diagnose the disease, which asserts itself in all cases by biopsies. Out of the nine cases, two had previously undergone an unnecessary laparotomy, and for three cases the diagnosis of rectal tumour established on rectal digital examination is twice set right by rectoscopic appearance and three times by biopsies. In the remaining five cases with mild intestinal symptoms and severe associated disease the diagnosis was only made with autopsies. A systematic rectoscopy with biopsies is indicated in all patients treated with antibiotics who present an unexplained diarrhea, collapsus or acute abdominal syndrom.