A polymerase chain reaction (PCR) procedure for detecting rickettsial DNA was developed and shown to be specific for Rickettsia rickettsii and R. conorii, the etiologic agents of Rocky Mountain spotted fever (RMSF) and Boutonneuse fever, respectively. Blood clots were obtained from nine confirmed RMSF patients and six controls and analyzed for the presence of rickettsial DNA by the PCR method. A defined region of the rickettsial genome was successfully amplified from seven of the nine clinical specimens tested; all six control specimens gave negative results. These findings indicate that R. rickettsii can be detected early after the onset of RMSF, possibly facilitating the decision regarding appropriate antibiotic therapy for some patients. Further refinement of PCR technology could make this procedure a mainstay in the clinical laboratory.