We examined the measles H gene using reverse transcriptase-polymerase chain reaction in peripheral mononuclear cells obtained from 4 pediatric and 2 adult patients with autoimmune hepatitis and 12 healthy children who had been infected with measles or vaccinated with an attenuated measles vaccine in the past. All patients were positive for the presence of the gene. Only one healthy control, who had been vaccinated two weeks before the study, was positive, while the other 11 controls were negative for the presence of the gene. The restriction enzyme patterns of the products in the pediatric patients were different from those observed in adults. The sequences of amplified products from pediatric patients coincided with the vaccine strain, whereas those from adults were different from the vaccine strain. The sequence of those from one of two adult patients was similar to those of the isolates in 1990 and later. Our results demonstrated that children with autoimmune hepatitis can have persistence of the vaccine strain in vivo for many years after vaccination.