Distribution of hepatitis B serological markers according to people's birthplaces and area of residence (urban/rural) was studied as a part of a field epidemiological research project carried out in Cássia dos Coqueiros, a small rural community of S. Paulo State, Brazil. The total prevalence of HBV markers was 7.7%, with rural showing a higher risk than urban inhabitants (9.8% as against 4.9%, respectively). Analysis of prevalence according to people's birthplaces revealed the highest value among those from other Brazilian states (15.8%), followed by people from other municipalities of the State of S. Paulo (9.2%). Those born in Cássia dos Coqueiros and particularly in Ribeirão Preto (the main city of the area, located 80 kilometers away), showed the lowest values of prevalence (5.2% and 2.5%, respectively). The importance of studying people's birthplaces when field epidemiological studies on hepatitis B are carried out is stressed. This variable is considered to be capable of exercising an influence on the natural history of the disease in a community, and may even explain differences in the distribution of markers in apparently similar populations.