The use of doxycycline, as a prophylactic antibiotic against scrub typhus was investigated in a prospective, randomized, double-blind study. Twenty volunteers were divided into two similar groups. Beginning three days before exposure to Leptotrombidium fletcheri chiggers infected with Rickettsia tsutsugamushi, and continuing for six weeks after exposure, one group received weekly 200-mg oral doses of doxycycline and the other group received a placebo. Nine of 10 doxycycline-treated subjects remained well during prophylaxis but developed antibody to scrub typhus, whereas nine of 10 subjects given the placebo required treatment for scrub typhus. Therefore, the efficacy of the regimen in preventing scrub typhus was 89% (eight cases prevented of nine expected). Ten days after successfully completing prophylaxis, eight of nine subjects reported minor self-limiting symptoms. A single dose of doxycycline was given on day 3 of illness to volunteers who developed scrub typhus. Such therapy was initially effective but was frequently followed by relapse and cannot be recommended.