This study was undertaken to estimate the prevalence of Brucellosis in patients attending the Armed Forces Hospital, Riyadh, Kingdom of Saudi Arabia. The method used was the standard agglutination test with doubling dilutions of sera from 10 to 5120. Further dilutions were made in 19 strongly positive ones at 5120 titre. A titre of 160 or more was considered significant for the diagnosis. Positive and negative control sera were used for each batch. The result of 1733 patient's sera tested showed that 153 (8.8%) were positive. Of these, 19 had titre greater than 5120. Two hundred and fifty five patients (14.7%) had titres ranging from 20 to 80. The prevalence of brucellosis in the patients studied by serological tests is 8.8% and control measures are necessary in the Kingdom of Saudi Arabia.