Rickettsia conorii, an obligate intracellular bacterium that infects vascular endothelial cells, is the etiologic agent of Mediterranean spotted fever. We correlated the results of 205 R. conorii blood and skin cultures for 157 patients and the results of 48 detections of R. conorii in circulating endothelial cells (CEC) for 41 patients with relevant serological, clinical, and therapeutic data. R. conorii was cultured from 40% of patients and 29.8% of samples. R. conorii was detected in CEC in 50% of samples, representing 46.2% of patients. When these calculations were limited to the samples from untreated patients prior to their seroconversion to R. conorii, the sensitivity of culture was 59%, whereas it remained at 50% for detection in CEC. We also performed PCRs for the detection of R. conorii on eight shell vial supernatants from positive cultures and on 43 blood samples. Only nonfrozen supernatants from fresh cultures were positive. The methods described in this report are suitable for use in all laboratories. Our findings suggest that for samples to be suitable for culture they must be collected prior to the initiation of an antibiotic regimen, as early as possible in the course of the disease, and be inoculated onto shell vials with minimal delay, if R. conorii is to be successfully isolated. For patients who have been treated or who have a delayed diagnosis, detection of R. conorii in CEC remains helpful.