HIV rapid testing may enhance the effectiveness of a mobile HIV/sexually transmitted disease (STD) screening clinic in at-risk populations who normally do not seek care. Our goal was to determine the usability and post-test counseling rates of rapid HIV testing services for clients tested on a mobile clinic. HIV Oraquick rapid HIV-1 testing (OraSure Technologies, Inc., Bethlehem, PA) (blood) was offered to clients seeking HIV/STI counseling and testing services from the street at predetermined locations in areas of high STD morbidity, drug use, and commercial sex work. Rapid test results were available on the same day at the van within 10 minutes. Disease intervention specialists (DIS) attempted to locate and counsel positive clients who did not stay for results. By comparison, when offered at the same time, 64.5% of clients preferred Oraquick to traditional serologic testing. The post-test counseling rate for clients tested for Oraquick was 89% for infected and 93% for uninfected. By comparison, 11% of infected clients and 40% of uninfected clients tested for traditional test were post-test counseled. Clients who tested for the traditional enzyme immunoassay (EIA) test were told to return to the van in 14 days for results and post-test counseling. In the adjusted model, we also found statistically significant differences comparing clients who choose Oraquick to traditional serologic tests. These data suggest that rapid HIV testing services may enhance the effectiveness of mobile STD/HIV clinics.