To overcome the limitations of current diagnostic methods for brucellosis, accurate and simple methods must be sought. In this study, sera of patients diagnosed with brucellosis were examined by polymerase chain reaction (PCR) for the detection of Brucella DNA. Results showed that an amplicon of 223 bp was obtained in 96% (24/25) of tested sera using primers derived from the nucleotide sequence of a gene encoding the 31-kDa Brucella abortus antigen. However, when serum DNA templates were examined by another PCR, an amplicon of 731 bp was obtained in 60% (15/25) of the tested samples using Brucella melitensis-specific primers derived from a polymorphic locus adjacent to the 3' end of IS711. The specificity of each PCR was demonstrated by inclusion of reference Brucella DNA strains and by DNA sequence analysis of PCR products. These results suggest that serum analysis by PCR is a convenient and safe method for rapid and accurate diagnosis of brucellosis and that B. melitensis is the leading cause of brucellosis in Saudi Arabia.