In inflammatory bowel disease (IBD), mucosal damage and loss of colonic function are regarded as major consequences of inflammation. Decreased colonic (Na⁺ + K⁺)-ATPase activities with diminished reabsorption of sodium and water have been found in active stages of ulcerative colitis. In this study, we report an inverse relationship between colonic (Na⁺ + K⁺)-ATPase activity and the degree of mucosal inflammation in 19 patients with IBD of mild to moderate disease activity. Various macroscopic and histologic types of mucosal lesions were differently associated with the (Na⁺ + K⁺)-ATPase activities. 5′-nucleotidase activity was not associated with the degree of mucosal inflammation or the kind of macroscopic or histologic lesions. Our findings support the view that, in contrast to 5′-nucleotidase, (Na⁺ + K⁺)-ATPase activity may better reflect the severity of mucosal damage and the degree of inflammation in IBD.