Using a new prototype (Olympus XPGIF 5.2/ N30) gastroscope, upper gastrointestinal endoscopy was safely performed under sedation on 99 infants (weights 0.9 to 10.1 kg). No complications occurred. Macroscopic and microscopic abnormalities were found in 60%, including abnormal duodenal biopsies in 47% of cases with protracted diarrhoea. Duodenal biopsy material was, however, of suboptimal standard for histologic assessment in 25% of the cases. In 52% of those under 3.5 kg diagnoses were made that would not have otherwise been possible and, in 75% of those in the Intensive Care Unit, abnormalities were found. The use of this new endoscope represents a significant advance in clinical practice.