Disease of the gastrointestinal tract occurs in up to 100% of patients who are infected with the human immunodeficiency virus (HIV) (Quinn et al, 1987). Gastrointestinal tract disease results from infections with a wide variety of pathogens (including HIV) and tumours (CDC, 1987; Nelson et al, 1988). Cytomegalovirus (CMV), like all herpes viruses, can cause primary, latent or chronic persistent infections. Life-threatening disease due to CMV is less common than subclinical infection in patients with acquired immunodeficiency syndrome (AIDS), and because of the overlap between asymptomatic CMV infection and serious CMV disease, prevalence data is difficult to interpret. Although 50% or more of patients with AIDS may have CMV viraemia, only 2.2% of 761 patients presenting to San Francisco General Hospital had CMV gastrointestinal disease (Quinnan et al, 1984; Jacobson and Mills, 1988). Other studies have shown a higher incidence of gastrointestinal CMV disease (between 5 and 10%) but this may reflect selection of patients and referral to gastrointestinal clinics (Connolly et al, 1989; Francis et al, 1989). CMV is now well established as a major cause of specific gastrointestinal syndromes including oesophagitis, gastritis, enteritis and colitis (Knapp et al, 1983; Balthazar et al, 1985; Freedman et al, 1985; Meiselman et al, 1985). It is now thought that CMV can also cause perianal and perioral disease. The clinical recognition of these syndromes is important in patients with HIV disease because of the difficulty in interpretation of CMV serology and CMV excretion in the immunosuppressed in general and in AIDS patients in particular (Pass et al, 1984; Brodie et al, 1985). Although CMV infection is usually disseminated, clinical manifestations occur at specific sites. The main criteria of active CMV disease are based on histological changes present on biopsy specimens. Modern endoscopic techniques make it relatively easy to obtain biopsy specimens from most of the gastrointestinal tract and thus histological evidence, along with the response to recently introduced therapeutic agents, helps to define the various syndromes of CMV gastroenteritis (Reichert et al, 1983; Chachoua et al, 1987).