Diphtheria is now uncommon in the United States, with outbreaks restricted to certain population clusters. Only 663 of 3,134 counties in the United States reported diphtheria in 1959–1970, and only 42 of these counties reported 10 or more cases during the 12-year period. The highest attack rates were in children one through nine years of age. In all age groups, the attack rates for minority racial groups were more than five times higher than in whites. Immunized children had lower case fatality rates than unimmunized children. The most frequently encountered Corynebacterium diphtheriae were biotype mitis in the North and South, while biotype gravis was most frequent in the West. Although there has been an irregular increase in incidence since 1965, with significant contributions resulting from a few outbreaks, there is no suggestion that a new type of control measure for diphtheria is needed. Eradication of carriers in households of patients with diphtheria and maintenance of immunization levels in children and adults have effectively limited diphtheria in the country at present, and further attention to these measures can be expected to be associated with further decreases in the number of counties and the number of cases of diphtheria in the country.