In two rural villages of the state of Puebla, Mexico, where Taenia solium pig cysticercosis is highly endemic, 120 pairs of young out-bred piglets were used to assay what proved to be an effective synthetic peptide vaccine against naturally acquired cysticercosis. Because the piglets used were all sired by one of three distinct studs in many different out-bred sows, the prevalence and intensity of infection, as well as degree of protection conferred by the vaccine, could be related to each of the three stud families (A-C). The highest prevalence was found in the C family (25%), whilst the prevalence of B and A families were 21.6 and 4.4%, respectively. Familial clustering of cases was even more conspicuous in vaccinated pigs than in not-vaccinated ones: seven of the nine cysticercosis cases that occurred in the vaccinated group belonged to the C family (7/26) and two to the B family (2/23), whilst the vaccine rendered the A family totally resistant (0/71). Parasite numbers were also higher in the C family in both nai;ve and vaccinated pigs. Familial clustering of cases and of large parasite numbers in naive and vaccinated pigs hint to the relevance of their genetic background in their innate and acquired resistance to cysticercosis.