In 1988, 22,796 Shigella isolates were reported to the Centers for Disease Control, the highest number since national surveillance was begun in 1967. From 1986 to 1988, isolation rates increased from 5.4 to 10.1 per 100,000 persons. Increased isolation of Shigella sonnei, primarily among children and young women, occurred throughout the United States in a manner similar to the nationwide increase that occurred during the early 1970s. The highest rates during 1987–1988 were reported from counties with relatively high proportions of urban, ethnic minority, and poor residents, groups traditionally at high risk. The greatest percentage increases in isolation rates, however, occurred in relatively wealthy counties with predominantly white residents. Between 1967 and 1988, the proportion of Shigella species isolated from persons >20 years of age increased 118%, while the proportion of the resident population in this age group increased only 16%. These data indicate a shift toward increased infection at older ages and the potential for periodic hyperendemic rates of shigellosis nationwide, which may be due to changing levels of immunity to S. sonnei.