To determine the prevalence of adenovirus and its serotypes in diarrhoeal stools in Saudi Arabia, 3,000 stool specimens were collected prospectively from subjects of all ages over a 28-month period. A total of 220 positive isolates were obtained. Fifteen serotypes were found, serotypes 40 and 41 being the most prevalent. Underlying disorders were found in the majority of patients, leukaemia being the most common single underlying condition. This investigation represents the first detailed study of the epidemiology of gastrointestinal adenovirus infections in Saudi Arabia.