Reported cases of Rocky Mountain spotted fever in the United States have been increasing since 1960 and reached an all-time high of 754 cases in 1974. Detailed clinical and epidemiologic information was obtained on 1522 (55%) of the 2757 cases reported in the 5-year period 1970 through 1974. Fifty-one percent of cases were confirmed by one or more laboratory test. The increase has occurred predominantly in the southeastern part of the United States. A comparison of laboratory-confirmed and unconfirmed cases suggests that a variety of febrile exanthems may be confused with Rocky Mountain spotted fever. Neither a history of tick bite nor rash was universally present, and both were significantly less frequent in fatal cases. The overall death-to-case ratio during this period was 6.8%. Death-to-case ratios were significantly higher for nonwhites (13.9) than whites (5.8), for male patients (8.2) than female patients (4.5), and for person older than 30 (13.9) than persons younger than 30 (5.4).