A case-control study was undertaken to determine for the first time the specific aetiology of animal to human transmission of brucellosis in Saudi Arabia. Cases consisted of all patients with brucellosis attending the primary care clinic of the Riyadh Al-Kharj Hospital programme in central Saudi Arabia. A sample of individually matched controls was selected concurrently from patients attending the same clinic for unrelated problems. Cases and controls responded to a 48 item questionnaire on exposure to established risk factors for brucellosis. Greatest risk was found to be associated with indirect contact with animals (the consumption of unpasteurized dairy products), as opposed to direct contact with animals. When specific animal products were considered, greatest risk was associated with the consumption of milk and laban (buttermilk), as opposed to cheese or uncooked liver. When specific animal species were considered, greatest risk was associated with products derived from sheep and goats as opposed to camels and cattle. When direct contact with animals was considered, the study found a very high risk associated with assisting in animal parturition, but no significant risk associated with other direct (unspecified) animal contact.