Intraluminal gastrointestinal pH was measured in seven patients with active ulcerative colitis (four male, three female). A radiotelemetry capsule was used, and its location was determined by fluoroscopy. Satisfactory measurements were obtained from six, in all of whom pH levels were normal in the stomach and small intestine. Three patients also had normal pH values in the colon. However, in the remaining three patients very low pH levels (lowest values 2.3, 2.9, and 3.4) were found in the proximal parts of the colon. Five of the seven patients, including the three with low pH in the colon, underwent colectomy. The mechanism behind the low intraluminal pH in some patients with ulcerative colitis is speculative. Increased fecal concentrations of lactate occur in active disease, but some of the pH values measured in our study were below the pKa value of lactate. The study demonstrates that very low intraluminal pH levels in the colon occur in some patients with active ulcerative colitis. This might be an indicator of severe activity of the disease.