A decrease in the colon’s ability to absorb Na⁺ and water is a major factor in the etiology of diarrhea in ulcerative colitis (UC). Vectorial ion transport in general is influenced by cell membrane voltage, which in turn depends upon K⁺ channel activity. Since colonic epithelial cells are depolarized in UC (Sandle et al. 1990), we used patch clamp recording to compare the activity of small (SK;~5pS), intermediate (IK; ~30pS), and large (BK; ~130pS) conductance K⁺ channels in the basolateral membranes of isolated human colonic crypts from control and UC patients. With informed consent, intact crypts were isolated (Sandle et al. 1994) from 4-5 rectosigmoid mucosal biopsies taken from 28 patients undergoing colonoscopic evaluation of functional bowel symptoms (controls), and 7 patients with moderately active UC. Cell-attached patches were obtained on the basolateral membrane of cells in the mid-third of the crypts, with NaCl Ringer solution in the bath and KCl Ringer solution in the pipette. Data are expressed as mean ± S.E.M., Student’s t test was used to analyse statistical significance (P < 0.05). In control patients, 37% of patches contained SK channels, 63% contained IK channels, and 5% contained BK channels. By contrast, in UC patients, 70% of patches contained SK channels, 43% contained IK channels, and BK channels were absent. Furthermore, SK channel activity (NPo, where N = number of channels in the patch, and Po = single channel open probability) was significantly greater in UC patients (0.93 ± 0.19) than in control patients (0.11 ± 0.02, P<0.001), whereas IK channel activity was significantly less in UC patients (0.31 ± 0.08) than in control patients (0.68 ± 0.08, P<0.05), and the low level of BK channel activity in controls (0.01 ± 0.01) disappeared in UC. Assuming K⁺ channels to be uniformly distributed within the basolateral membrane, overall basolateral membrane K⁺ conductance in UC patients (46.6%, 53.4% and 0% of which was related to SK, IK, and BK, respectively) was 63% (P<0.02) lower than control patients (2.4%, 89.2%, and 8.4% of which was related to SK, IK, and BK, respectively). UC is associated with substantial shifts in the pattern of basolateral membrane K⁺ channel expression/activity in colonic crypt cells. This decreases overall basolateral K⁺ conductance by 63%, which provides an explanation for the epithelial cell depolarization that underlies defective Na⁺ absorption in this disease.