In a gastric mass survey with photofluorography performed on 10,605 male office workers over 40 years of age, 456 cases of duodenal ulcer (4.3%) were detected. These cases were scheduled to be followed up every 6 months with endoscopy and without any anti-ulcer drugs. Two hundred and seventy-six of the cases, including 169 cases with craters and 107 cases with scars, were followed up for 2 years. Forty-one of 169 craters (24.3%) had healed, and 21 of 107 scars (19.6%) had relapsed at 6 months. At 24 months 36 of 169 craters (21.3%) had healed, and 31 of 107 scars (29.0%) had relapsed. Sixty-four and a half per cent of the ulcers that showed crater at entry remained at the crater stage, and 62.6% of ulcer scars at entry remained healed at every endoscopy during the trial period. With regard to the cases' background, cigarette smoking adversely affected the natural history of the duodenal ulcer. However, years after onset, previous treatment, history of overt bleeding, and alcohol and coffee consumption did not affect the present ulcer activity.