One hundred consecutive out-patients with duodenal ulceration from a hospital and a gastroenterological clinic were tested with the Minnesota Multiphasic Personality Inventory (MMPI). This was carried out in order to investigate whether neuroticism or other personality disorders were characteristics of duodenal ulcer patients, and whether the presence of such possible personality disorders might influence the prognosis of the disease. Neuroticism occurred in 53% of the patients, but only in 5% of controls (P less than 0.0001). Overall, personality disorders were present in 69% of the patients compared with 30% of the controls (P less than 0.0001). Neuroticism was connected with a high frequency of relapse (P less than 0.05) whereas failure of spontaneous ulcer healing had no certain relation to personality disorders. Patients with non-neurotic personality disorders had more frequently suffered stressful life events before entrance to the study (P less than 0.05) and, like the neurotic patients, they had lower ego-strength to cope with such events (P less than 0.05). The results indicate that personality assessments make it possible to distinguish between subgroups of duodenal ulcer patients with different course of the disease.