The rectal mucosa-associated flora (MAF) of patients with ulcerative colitis has been studied in 25 patients with newly diagnosed disease, 20 with relapse of existing disease, and 44 who were in remission. Patients with active disease were re-examined twice during treatment. The MAF was simpler and less dense than the microflora of faeces. Obligate anaerobes usually predominated in the MAF although the ratio of obligate anaerobes to facultative species was lower than that found in faeces. Viable counts of the total flora and of its constituent genera varied considerably between patients. Counts of the total flora, of obligate anaerobes (including bifidobacteria, eubacteria and clostridia), and facultative organisms and micro-aerobes (enterobacteria and lactobacilli) were reduced in patients with active disease compared with those with inactive disease; corresponding carriage rates were also lower. Counts and carriage rates increased during treatment and approached those found in quiescent disease. The alterations in the MAF were especially marked in patients experiencing their first attack of ulcerative colitis. The relationship between these alterations and the aetiology and pathogenesis of this disease remains unclear.