The characteristics of peptic ulcer and non-ulcer dyspepsia in young men were studied in 202 consecutive conscripts who attended Central Military Hospital in Helsinki because of long-standing upper abdominal complaints. Active peptic ulceration (APU) was found in 48 patients, inactive peptic ulcer disease (IPU) was diagnosed in 77 patients, non-ulcer dyspepsia (NUD) was diagnosed in 52 patients. In 25 cases the reason for symptoms was another disease, and these patients were excluded from the study. A control series (CON) consisted of 30 symptomless healthy young male volunteers. The likelihood of discriminating between peptic ulcer disease and non-ulcer dyspepsia in a young male patient with dyspepsia are indicated by odds ratios (OR) and its 95% confidence limits (CL 95). Active peptic ulcer disease differs from NUD, e.g., by 1) presence of antrum gastritis, OR 41.5 (CL 95: 10.1-171), 2) Helicobacter pylori in the gastric mucosa, OR 31.0 (7.4-130), 3) Lewisa+ phenotype, OR 8.9 (1.7-45.4), 4) serum pepsinogen I (S-PGI) greater than 100 micrograms/l, OR 4.6 (1.7-12.4), 5) non-secretor status, OR 4.3 (1.6-11.6), and 6) O-blood group, OR 3.0 (1.2-7.7). In conclusion, the status of gastroduodenal mucosa, gastric secretion pattern and distribution of some genetic markers in patient series indicate that young onset peptic ulcer and non-ulcer dyspepsia are two separate entities. Helicobacter-positive antrum gastritis is the best determinant of ulcer risk, but also high S-PGI, Lewisa+ phenotype, non-secretor status and O-blood group are signs of increased risk of peptic ulcer.