Background: The classical manifestation of tsutsugamushi disease was attributable to Leptotrombidium akamushi; the peak incidence occurred in August. Now, for reasons unknown, the disease has peak incidences in May and November, the new vectors being L. pallidum and L. scutellare, respectively. Methods: Epidemiologic data on tsutsugamushi disease occurring in Japan over a period of 44 years (1950-1993) are analyzed. Results: The number of cases gradually declined from 116 in 1950 to three in 1969. The number increased again to 941 in 1990. The disease is endemic in almost all parts of Japan, but it exhibits different rates of morbidity. Miyazaki and Kagoshima, which are prefectures in the southern part of the country; Chiba and Kanagawa, which are near Tokyo; and Akita and Niigata in the northern part of Japan are highly endemic. Three quarters of patients are between 50 and 70 years of age, range <4 to >80 years. Conclusions: Prognosis is good when treatment with tetracycline or doxycycline in a traditional dose is given for 10-14 days. Administration should be as early as possible. Preventive medication is not considered necessary.