Chronic ulcerative colitis is considered a "premalignant" lesion in patients with chronic and total colonic disease. In organ culture, we have studied the mucosa from normal human colon, and from patients with ulcerative colitis. These tissues were maintained for up to 20 hr in organ culture, and a measure of proliferative activity was studied. Since phosphodiesterase inhibitors decrease thymidine incorporation into rabbit colon DNA, we studied this parameter to see whether premalignant mucosa responded differently from mucosa that was not premalignant. Thymidine incorporation in normal mucosa was inhibited maximally by 5 mM theophylline (72 ± 4% of control without theophylline). The effect of theophylline on the incorporation of thymidine into DNA was studied in 26 patients with ulcerative colitis in remission, 10 with only rectal or left-sided involvement for up to 15 yr, 8 with universal colitis for less than 10 yr, and 8 with universal colitis for over 10 yr. When all patients with left-sided colitis were studied, the incorporation of thymidine was 85 ± 6% of the control value without the drug. However, when patients in this group were divided, theophylline inhibited the group with less than a decade of disease (70 ± 4%), but not those with disease for over 10 yr (97 ± 3%). Mucosa from the group with universal colitis for less than 10 yr responded to theophylline (69 ± 3% compared with no drug). The group with universal colitis for over 10 yr showed no significant inhibition (95 ± 5% of control) despite the lack of histologically premalignant mucosa in the cultured samples. Three ulcerative colitis patients with cancer and premalignant changes in their colon were also studied. None of the morphologically premalignant areas demonstrated inhibition of thymidine incorporation by phosphodiesterase inhibitors (84-163% of control). We conclude that patients with longstanding colitis in remission demonstrate a defect in the modification of thymidine incorporation into DNA by phosphodiesterase inhibitors, regardless of the extent of their disease. Moreover, this biochemical defect can be detected in tissues in which premalignant morphologic changes are not evident. These data suggest that the duration of inflammatory change, more than the extent of the disease within the colon, may be the more important factor in leading to the observed alteration in control of DNA synthesis.