Twenty-five children with radiologically confirmed primary peptic ulcers were investigated. All patients suffered from duodenal ulcer and there was an unexpected female preponderance. The frequency of introverted personalities was greater in the patients than in the controls. Five patients suffered from psychiatric disorders, three had histories of suicidal attempts and three reported homosexual experiences. Six patients had nicknames. Eight patients had been operated on for appendicitis. The above parameters were negative in all control cases. The patients had lower mean IQ, worse adaptation to school, more anxious and overprotective parents, higher frequency of faddiness in food and lower frequency of nail-biting than the controls. The onset of symptoms was preceded by psychotraumatic events in eight cases. The findings suggest that the manifestation of the genetic background of duodenal ulcer is strongly influenced by environmental factors and factors associated with the personality of the patients.