OBJECTIVES To measure the prevalence of gonorrhea, chlamydia, and human immunodeficiency virus (HIV) infection among emergency department (ED) patients who accept screening, and to assess treatment outcomes and risks for infection. METHODS Research staff offered voluntary testing for gonorrhea and chlamydia (by urine transcription-mediated amplification) and HIV (by enzyme immunoassay/Western blot of oral mucosal transudate) to ED patients. Pediatric (15-21 years) and adult (22-29 years) patients were eligible for gonorrhea and chlamydia testing; patients aged 15-54 years were eligible for HIV testing. The authors surveyed behavioral risks of patients accepting HIV testing. RESULTS From November 2003 to May 2004, 497 of 791 eligible pediatric patients (63%) and 1,000 of 2,180 eligible adult patients (46%) accepted screening for gonorrhea, chlamydia, and/or HIV. There were 41 patients infected with gonorrhea, chlamydia, or both among 380 pediatric patients (10.8%) and 11 of 233 adult patients (4.7%); 14 of 52 patients (27%) were treated presumptively by ED clinicians. Through study efforts, 33 of the 38 remaining patients were treated (90% overall treatment). Eight HIV infections were diagnosed: seven of 969 adult patients (0.7%) and one of 459 pediatric patients (0.2%); five HIV-infected patients (63%) received test results, and three (38%) attended an HIV clinic. Gonorrhea or chlamydia infection in pediatric patients was associated with multiple sex partners, same-sex intercourse, and suspicion of sexually transmitted diseases by the ED clinician. CONCLUSIONS The high prevalence of gonorrhea and/or chlamydia infection among pediatric ED patients tested supports consideration of expanded screening. Targeted HIV screening with rapid tests merits exploration in the authors' ED, given the low-moderate numbers of patients identified through screening, receiving test results, and linked to care.