Dramatic changes in the size and racial composition of the nation's largest urban school districts are indicators of an increasingly multiracial and highly segregated society. Data were analyzed from the 1986-87 and 1984-85 Department of Education computer tapes containing the enrollments by race of 36,000 schools from 3,400 districts nationwide. All districts that had more than 19,000 students and at least 50 students of any minority group enrolled were examined. Major declines in overall White enrollments have occurred, reflecting changes in the national birthrate. A number of the largest districts have experienced particularly acute White declines. Contrary to earlier expectations, White districts losing White enrollment have seldom become virtually all Black. Most are becoming more multiracial, showing heavy increases in Hispanic and Asian enrollments. These changes do not appear to be linked to any particular type of school desegregation plan or neighborhood schools approach. Trends in racial segregation patterns from 1967 to the present show that Black and Hispanic students in big city school districts are usually highly segregated from White students. For Blacks, the data show pattern changes that appear to be related to desegregation orders, the initial racial composition of the district, and demographic changes. Hispanics in central cities with significant Hispanic enrollments have become more segregated with virtually no exceptions. Extensive statistical data are included on 22 tables. (FMW)