A retrospective survey was undertaken to provide the first reported estimate of the incidence of human brucellosis in Saudi Arabia. The study population was unusually well defined and consisted of all individuals resident in the Riyadh and Al Kharj regions and registered for treatment in the Riyadh-Al Kharj Hospital Program. Cases satisfying predetermined case criteria were identified initially from a retrospective review of hospital laboratory records, and this was supported by a review of individual medical case-notes. Brucellosis was found to be much more common in Saudi nationals than expatriate nationals. Among Saudi nationals the study demonstrated a remarkable increase in brucellosis with increasing age, and a higher incidence amongst women than men in some age groups. This was felt to have been due either to an increased exposure to infected livestock, or to an increased susceptibility to the disease in women, and with increasing age. There was a seasonal fluctuation in the occurrence of brucellosis with the largest number of cases occurring in spring and summer.