Between December 23, 1975, and March 31, 1976, 169 cases of measles were reported from Defiance County, Ohio, a small rural county in the northwest corner of the State. The outbreak spread from a single junior high school basketball player to eventually involve 19 of the 28 county schools. Among the affected schools, measles attack rates varied from 0.3-7.2% with a mean of 2.0%. A likely source of illness was determined for 160 of the 169 cases (95%). Intraschool transmission was most common, accounting for 97 of the 169 cases (57%) followed by sibling contact for 23 cases (14%). The pattern of measles spread was complex and would have been difficult to predict in advance even if surveillance systems reported each case the day it occurred. A control program held between February 2 and February 20, 1976, vaccinated 5145 of the 11,114 (46.3%) county schoolchildren. Forty-four cases of measles occurred 4 or more days following school clinics, 22 (50%) in children who requested measles vaccine at school clinics, 17 of whom were actually vaccinated. Most of the other cases occurred in students whose parents thought their children to be protected. Measles is a disease which spreads rapidly in a complex pattern over wide geographic areas. A control program vaccinating a large proportion of the children without definitive history of adequate vaccination or disease was apprently effective in curtailing the outbreak.