This report examines the significance of the ethnic composition of Dade County (Florida) high schools for the academic achievement and dropout rate of its students. The following statistical data were analyzed using multivariate analysis for all 76,830 students enrolled in grades 9-12 in 1989: (1) age; (2) sex; (3) grade; (4) ethnicity; (5) enrollment in English as a Second Language (ESL); (6) school identification; (7) reading and mathematics achievement test scores; and (8) dropout rates. The following findings are included: (1) a high concentration of Black students in a school is associated with an overall decrease in achievement; (2) a high concentration of Hispanic American students is associated with an overall gain in achievement; (3) Black students tend to have higher dropout rates in schools with a high concentration of Black students; and (4) Hispanic American students tend to have lower dropout rates in schools with a high concentration of Hispanic American students. Eighteen tables of statistical data are appended. (FMW)