The prevalence of antral and body gastritis was determined in 30 duodenal ulcer patients and in 143 of their first-degree relatives, and compared by conventional mathematical and stochastic analyses with data on gastritis in a representative Finnish population sample. For conventional analysis, the controls for the duodenal ulcer patients and for the duodenal ulcer relatives, were matched for age and sex. For stochastic analysis, the duodenal ulcer patients and their 99 siblings were compared with the total control population of 434 subjects. The prevalence of gastritis affecting mainly the antral mucosa, and both antral and body mucosa to a similar extent was significantly higher in duodenal ulcer patients than in both controls and in relatives. The prevalence of antral and body gastritis in DU relatives and their controls was similar. However, the prevalence of subjects with normal antral and body mucosa was significantly lower. Stochastic analysis revealed more rapid progression of antral gastritis with age in the duodenal ulcer patients than in their siblings or controls and less rapid progression of body gastritis. The overall progression of antral and body gastritis was similar in DU siblings and their controls, but a dichotomy in the mean antral gastritis score of DU sibships was found, indicating high and low antral gastritis liability subgroups. The mean score of DU sibships having a mean age of less than 50 years behaved dynamically like DU patients, while the mean scores of sibships with a higher mean age had a low liability to develop antral gastritis. Most duodenal ulcer siblings who themselves had a duodenal ulcer, ulcer scar or duodenitis were found in the "high antral gastritis liability" subgroup.