A previously healthy man presented with a five day history of high fever and headache, later followed by rash and the appearance of jaundice. On the second hospital day, he suddenly developed seizures, lapsed into a coma, and died. Polymerase chain reaction (PCR) amplification revealed a 434 base pairs DNA fragment common to the genome of typhus and spotted fever group rickettsiae in the patient's blood (estimated at about 1 x 10(2) organisms/ml), and to a lesser degree in the cerebrospinal fluid. However, serological tests for rickettsiae remained negative. PCR techniques may confirm the diagnosis at an early stage, even though the rickettsemia may be minimal and the patient seronegative.