Women who seek medical care following sexual assault are usually evaluated and treated in an emergency department (ED). Therefore, EDs can be an important source of sexual assault surveillance data. The authors compared the incidence of sexual assault presenting for emergency care in a single county during July to November of 1974 and 1991. Participants included all female sexual assault victims aged 14 and older who presented for ED evaluation. Treating physicians prospectively collected data using standardized forms. The z statistic was used to compare sexual assault incidence. There was a 60% increase in the incidence of sexual assault victims presenting for emergency care in 1991 compared to 1974, primarily due to an increase in the incidence of women presenting to the ED after rapes by known assailants. In contrast, the annual incidence of reported stranger assaults was similar in the two study years.