In order to understand whether the serum pepsinogen I (PGI) levels of non-ulcer dyspepsia (NUD) patients differed from those of healthy controls and which kinds of factors might influence these levels of the former, serum PGI levels were determined by radio-immunoassay in 120 healthy controls and 361 NUD patients. The mean serum PGI level was significantly lower in NUD patients than in the controls (75.8 +/- 38.2 ng/mL versus 95.9 +/- 39.3 ng/mL; P less than 0.0001); in NUD patients, it was significantly higher in males than in females (79.6 +/- 40.4 ng/mL versus 63.2 +/- 26.4 ng/mL; P less than 0.001). However, corrected serum PGI levels according to the lean body mass disclosed no sex difference. Nevertheless, there was a weak correlation between the ages of patients and serum PGI levels (r = 0.403). The serum PGI levels in blood groups A, B, AB and O did not differ significantly. Smoking might be a decisive factor, for the mean serum PGI level was significantly higher among the smokers than among the non-smokers (86.7 +/- 44.6 ng/mL versus 70.6 +/- 33.7 ng/mL; P less than 0.001). It is concluded that the mean serum PGI level of NUD patients, while affected by sex, age, smoker status and the body size, is lower than that of the healthy controls. Without endoscopy, assay of serum PGI level has no place in the diagnosis of dyspepsia because of the wide overlapping of serum PGI levels between NUD patients and patients with other common gastric lesions.