Thirteen patients of duodenal ulcer, below 20 years of age, ten age and sex matched controls and 17 first probands were studied to find out (i) the special characteristics of this duodenal ulcer subtype and (ii) the serum alpha-1-antitrypsin (alpha-1-AT) activity in these patients and their first probands. Basal and maximal acid output, and serum level of alpha-1-AT were measured, and serum agar gel electrophoresis done, in the above subjects. Special characteristics like positive family history, and high basal (p less than or equal to 0.01) and maximal (p less than or equal to 0.001) acid output were noted in these patients. Serum alpha-1-AT was found to be deficient in nine patients. Five first probands also had the heterozygous level of the enzyme deficiency. Thus, patients of early onset duodenal ulcer seem to have special characteristics with alpha-1-antitrypsin deficiency in them as well as in their first probands.