A total of 394 children, aged 0-14 years, referred to a paediatric department were investigated for the occurrence of Clostridium difficile, its cytotoxin in faeces, and its clinical significance in a prospective study over one year. Of the children in the investigation, 337 suffered from gastrointestinal diseases or had been treated with antibiotics prior to the investigation. Twenty-four percent of these children had Cl. difficile in one or more faecal samples and toxin was demonstrated in half of these patients. In 18/148 (12%) of the patients with acute gastroenteritis Cl. difficile was isolated as the only pathogen. In contrast, among 57 control children with no gastrointestinal symptoms and no prior antibiotic treatment significantly fewer harboured Cl. difficile (p less than 0.01). Cl. difficile was isolated with similar frequency in children with or without prior antibiotic treatment. Isolation of Cl. difficile was significantly higher in patients under one year of age (p less than 0.001). In 78% of the patients with Cl. difficile the bacteria were found in faeces during the initial two days of hospitalization indicating that most of the cases were community acquired. The occurrence of Cl. difficile was not influenced by seasonal variation and the distribution between sexes was equal.