Campylobacter spp. were the organisms isolated most frequently from 29 consecutive patients admitted with acute diarrhoea to an infectious disease unit. Rectal biopsies taken from 21 of the patients were abnormal in all but four, and in the patients with campylobacter infection there was a characteristic proctocolitis in each case. The histopathology was similar to that described for salmonella and shigella infections but clearly different from typical ulcerative colitis and Crohn's disease. Therefore in patients with acute diarrhoea we suggest that selective culture for Campylobacter spp. should be made a routine and that rectal biopsy has an important diagnostic role, particularly in patients with negative cultures.