The prevalence of the hepatitis A virus in a population determines the degree of morbidity associated with this illness, that is, the higher the morbidity, the lower the prevalence. This study aims to obtain the prevalence of total antibody to the hepatitis A virus in children, 5 and 8 years of age, and in adolescents, 14 years of age. The study was based on two samples: the serum of 64 healthy five-year-olds and 76 healthy eight-year-olds living in the proximity of the São Marcos Hospital in Braga and the serum of 311 adolescents, aged 14, from a total population of 536 adolescents attending schools in Braga, North of Portugal. The samples were collected in 1999 for the adolescent group, in 2000 and 2001 for the group of the five-year-olds and in 2002 and 2003 for the group of the eight-year-olds. None of the persons involved had been vaccinated with the hepatitis A virus vaccine. The Enzyme Linked Fluorescent Assay method was used to measure the serum total antibody to the hepatitis A virus. The prevalence of total antibody to the hepatitis A virus was 1.6% at 5 years of age, [95% confidence intervals (CI), 0-4.7%]; 3.9% at 8 years of age, [95% CI, 0-8,4%]; and 32.5% at 14 years of age +/- 6 months, [95% CI, 27.3-37.7%]. The prevalence of total antibody to the hepatitis A virus in this population revealed lower natural immunity. The results obtained for the five and eight-year-olds were the first Portuguese results of low endemicity to the hepatitis A virus.