Nine members of a family with a high incidence of duodenal ulcer disease were studied by interview, examination of hospital records, endoscopy, and antral biopsy. Helicobacter pylori was confirmed by CLO test, histology and culture. DNA extraction from pure isolates of H pylori was possible in six family members and strain typing was performed by restriction fragment length polymorphism. DNA restriction digestion was followed by vacublotting and then DNA hybridisation, using a cDNA probe complimentary to H pylori rRNA cistrons. Eight of the nine family members were H pylori positive by CLO test and histology. Five had duodenal ulcer disease. Three family members (one from each generation) harboured clonal variants of a single parent strain of H pylori but only two had duodenal disease. The other three members harboured different strains. Intrafamilial clustering of clonal variants of H pylori occurs in some duodenal ulcer disease families. Family members however, may develop duodenal disease irrespective of the colonising strain.