This population-based study was carried out using the medical records linkage data resource for the population of Rochester, Minn, at Mayo Clinic. There were 132 newly diagnosed cases of psoriasis identified during a 4-year period (1980 through 1983); 88% of the cases had been seen and diagnosed by a dermatologist. The overall crude incidence rate was 57.6 per 100,000 population; for men and women, the rates were 54.4 and 60.2, respectively. The overall sex- and age-adjusted (1980 US white population) incidence rate was 60.4 per 100,000 person-years. The highest rate of occurrence (112.6) was in the 60- to 69-year-old age group. Most of the cases of psoriasis diagnosed in this study (58%) were mild, and the patients had psoriatic lesions on less than 10% of their body. There are no other published incidence rates for this condition with which to make comparisons.