Acquisition of Helicobacter pylori occurs mainly in childhood. However, the mode of transmission remains unclear. To help elucidate this, 100 children attending for upper gastrointestinal endoscopy were investigated for the presence of H. pylori at various sites. H. pylori was detected in antral gastric biopsies by the rapid urease test (13 patients), culture (13 patients), histology (15 patients) and PCR (20 patients). Gastric juice was positive for H. pylori in 3 patients by culture and 11 patients by PCR. The dental plaque from 68% of gastric biopsy-positive patients (as determined by culture or PCR) and 24% of gastric biopsy-negative patients was positive for H. pylori by PCR. The presence of H. pylori in dental plaque was significantly associated with the presence of this organism in the stomach. H. pylori was detected by PCR in the faeces of 25% of gastric biopsy-positive children sampled. H. pylori was not cultured on any occasion from the oral cavity or faeces. The evidence from this study suggests that oral-to-oral transmission may be a possible mode of spread of H. pylori in children.