To determine the prognostic importance of microscopic rectal inflammation we followed up 82 patients (aged 21 to 78 years, 44 men) with chronic quiescent ulcerative colitis over 12 months. At trial entry each patient underwent a rectal biopsy and sections were graded independently by two histopathologists. A chronic inflammatory cell infiltrate of varying severity was present in all biopsy specimens, and 58% had crypt architectural irregularities. In addition, 32% had evidence of acute inflammatory activity: 28% acute inflammatory cell infiltrate, 11% crypt abscesses, and 22% mucin depletion. Agreement between the two histopathologists for the presence of each of these features was 94% (90-98%). During the 12 month follow up 27 patients (33%) relapsed after a mean interval of 18 weeks (range 3-44 weeks). Relapse rates were unrelated to duration or extent of disease or to the type of maintenance drug treatment. In patients with an acute inflammatory cell infiltrate 52% relapsed, whereas in the absence of such an infiltrate only 25% relapsed (p = 0.02). Similarly, relapse rates were higher in the presence of crypt abscesses (78% v 27%, p less than 0.005), mucin depletion (56% v p less than 0.02), and breaches in the surface epithelium (75% v 31%, p = 0.1). The presence of a chronic inflammatory cell infiltrate or crypt architectural irregularities, however, bore no relation to the frequency of colitis relapse.