There have been indications that human brucellosis is widely distributed in Saudi Arabia. In order to assess the situation in the south, and as a part of a nationwide prevalence survey, a sample of 4900 subjects was randomly selected for a house-to-house survey. Investigations included an interview, clinical examination and blood sampling for antibody titre determinations. Blood samples were first screened for Brucella antibodies by a microplate agglutination test to measure the exposed rate. Reactive sera were further analysed by the standard tube agglutination and 2-mercaptoethanol tests. A total of 4794 completed the study. Results of laboratory tests indicated that a significant proportion of the population in the southern region (19.2%) had serological evidence of exposure to Brucella antigen, and 2.3% had active disease. Direct contact with domestic animals and consumption of raw products of animal origin were identified as the main risk factors.