A polymerase chain reaction (PCR) for amplifying ribosomal DNA of Rickettsia rickettsii was performed on blood clots and urine samples from 10 patients with suspected Rocky Mountain spotted fever (RMSF) and five controls with nonrickettsial diseases. The results of this PCR-based procedure were positive in four of the five patients with probable RMSF, but reamplification was required in three patients. Rickettsia rickettsii was grown from the blood of two of these four patients. The urine from one patient was also PCR-positive. These results confirm earlier findings that the PCR can detect R. rickettsii, but the need for reamplification indicates that the lack of sensitivity is a serious limitation in the usefulness of the PCR as a clinical diagnostic test.