A population-based epidemiologic study of pityriasis rosea was developed for Rochester, MN, for a recent 10-year period. Nine hundred thirty-nine patients with pityriasis rosea (340 male patients and 599 female patients) who resided within Rochester city limits at the time the diagnosis was made were included. The average annual incidence rate of this disease was 172.2 per 100,000 person-years (158.9 per 100,000, adjusted to the 1970 U.S. white population). Female patients predominated by a margin of 1.5:1.0. More than 75% of the patients were between the ages of 10 and 35 years, with a mean age of 22.7 years and a range of 10 months to 78 years. There was a recurrence of the illness in 1.8% of the patients after an average of 4.5 years of follow-up. The highest rate was noted in 1972, following which there was a steady and significant decline in incidence. The incidence of the disease was significantly higher in the colder months than in the warmer season. Patients in twenty-one close-contact pairs experienced the disease at varying intervals. Recent past infections or history of atopy was noted in nearly one third of the cases.