In late January 1985, a measles outbreak occurred at a community hospital in Columbia county, Florida. The outbreak spread throughout the county and to two neighboring counties (Alachua and Marion), resulting in 79 cases with a 29% hospitalization rate. Hospitals represented the site with the highest frequency of transmission. At the Alachua county hospitals, where strict respiratory isolation measures were taken, no secondary cases occurred among hospitalized patients. Two independent risk factors existed for hospitalization: measles exposure in a hospital setting (P <0.05) and nonvaccination (P <0.00l). Of the total measles cases, 24% were under the age of 16 months and 47% of those aged 16 months or older had a history of appropriate vaccination. Columbia county, which experienced 86% of the cases, had a 5% frequency of unvaccinated students compared to 0.6% frequency at Alachua (P <0.00l) where only 10% of the cases occurred. This outbreak demonstrates the role of uncontrolled nosocomial transmission of measles in the propagation of a community outbreak.