Serum group I pepsinogen (PG I) levels have been determined by radioimmunoassay in 15 patients without, recurrent ulcer after vagotomy and either a gastric resection or a drainage procedure. The mean (+/-SE) levels were 151.8 +/- 16.9 ng per ml in the patients with recurrent ulcer and 79.7 +/- 9.8 ng per ml in those without recurrence (P less than 0.001). A recurrent ulcer was present in 6 of 7 patients with an elevated serum PGI (greater than 175 ng per ml) but not in any of 10 patients with a low serum PGI (less than 50 ng per ml). The correlation between serum PG I and peak acid output (PAO) was statistically significant in patients with recurrent ulcer (pi=0.815, P less than 0.001) and in those without recurrence (r= 0.540, P less than 0.025). In patients with recurrent ulcer, a serum PG I level within the normal range (50 to 175 ng per ml) was uniformly associated with a PAO of more than 10 mEq per hr. In contrast, of 10 patients without recurrent ulcer and a normal serum PG I, eight had a PAO of less than 10 mEq per hr. The reason for the discordant results in the two groups of patients is not certain.