The behaviour of acid secretion, serum pepsinogen I and II (PG I and PG II) and morphology of the gastric mucosa were analyzed in 59 duodenal ulcer probands (DU probands), their 199 first-degree relatives and 228 control subjects. DU probands had as a rule antral gastritis with normal or slightly altered corpus mucosa, and higher mean peak acid output (PAO), PG I and II levels than their relatives and controls. Sibs of DU probands differed from their controls mainly with regard to morphology which showed features characteristic of DU probands, i.e. antral gastritis with normal or slightly altered corpus mucosa. Moreover, high PAO levels were found highly significantly more often in sibs of DU probands (13%) than in controls (6%). Likewise, the prevalence of endoscopic signs of active or past duodenal ulcer were present in sibs highly significantly more often than in controls and they accumulated in the subgroup of sibs with high PAO or PG I levels. It seems probable that the occurrence of high PAO and PG I levels in sibs of DU probands can be considered as signs of increased liability to duodenal ulcer. PAO and PG I were as expected significantly higher in male than in female probands, relatives and controls. Exclusion of cases of corpus gastritis decreased the levels but the sex difference persisted. PAO, PG I and II revealed a significant increase of the levels in middle age followed in older age in case of PAO and PG I by a significant decrease. The decrease was abolished when the cases of corpus gastritis were excluded suggesting an effect of gastritis. However, the earlier increase of the levels remained virtually unaffected although there was a uniform decrease of the mean values. This suggests the participation of factors unrelated to gastritis. The nature of the factors remains unknown, but literature data and data derived from our recent study suggest involvement of anatomical factors such as an increase in the size of acid and PG I secreting area.