Elevated levels of luminal nitrite and a lowered luminal pH were found in 77 percent of patients with acute ulcerative colitis. No luminal nitrite was found in healthy control subjects. Nitrites are a secretory product of activated macrophages and neutrophils of the lamina propria, whereas the lowered luminal pH is due to diminished bicarbonate formation by impaired colonocytes. A hypothesis is put forward that nitrites, lowered pH, and bacterial amines are conducive to formation of carcinogenic n-nitroso compounds, which reflect a cancer risk in patients with ulcerative colitis dependent on the type and extent of inflammatory cell activation as well as metabolic impairment of colonic epithelial cells.