To determine whether the serum pepsinogen I (PG I) level would be a suitable marker for selecting patients at risk for duodenal ulcer recurrence and, thus, would benefit from maintenance therapy, we treated duodenal ulcer patients with H2-receptor antagonists. After healing 140 ulcer patients we assessed the recurrence rate at 1 year with and without maintenance therapy. The annual recurrence rates in duodenal ulcer patients with hyper-PGI (95 ng/ml or more), with 66 ng/ml less than or equal to PGI less than 95 ng/ml, and with PGI less than 66 ng/ml were 87.0%, 27.3%, and 17.9%, respectively, when they did not receive maintenance therapy. In patients with hyper-PGI the recurrence rate was significantly lower in patients receiving maintenance therapy than in patients not receiving maintenance therapy, whereas in patients with PGI less than 66 ng/ml the recurrence rate was as low as 20% regardless of maintenance therapy. These results indicate that maintenance therapy with half the dose of H2-receptor antagonist is not required by patients with PGI less than 66 ng/ml, whereas those with hyper-PGI may be good candidates for long-term maintenance therapy.