Objectives Adolescents and young adults are at high risk for sexually transmitted infections (STIs). We previously reported an increase in STI testing of adolescents in our ED by obtaining a sexual history using an Audio-enhanced Computer-Assisted Self- Interview (ACASI). We now examine associations among demographics, sexual behaviour, chief complaint and willingness to be tested. Methods This was a prospective study conducted in a paediatric ED between April and December 2011. After triage, eligible patients between 15 and 21 years presenting with non-life-threatening conditions were asked to participate in the study. Consenting participants used an ACASI to provide their demographic data and answer questions about their sexual history and willingness to be tested. Our primary outcome was the association of demographics, chief complaint and ACASI recommendation with the participant's willingness to be tested. Results We approached 1337 patients, of whom 800 (59%) enrolled and completed the ACASI. Eleven who did not answer questions related to their sexual history were excluded from analysis. Of 789 participants, 461 (58.4%) were female and median age was 16.9 years (IQR 16.0-17.8); 509 (64.5%) endorsed a history of anal, oral and/or vaginal intercourse. Disclosing a sexual history and willingness to be tested did not differ significantly by gender. 131 (16.6%) had a chief complaint potentially referable to an STI; among the 658 participants with non-STI-related complaints, 412 (62.6%) were sexually active, many of whom disclosed risky behaviours, including multiple partners (46.4%) and inconsistent condom use (43.7%). The ACASI identified 419 patients as needing immediate STI testing; the majority (81%) did not have a chief complaint potentially related to STIs. 697 (88.3%) participants were willing to receive STI testing. Most (94.6%) of the patients with STI-related complaints were willing to be tested, and 92.1% of patients with a recommendation for immediate testing by the ACASI indicated a willingness to be tested. Conclusions Adolescents were willing to disclose sexual activity via electronic questionnaires and were willing to receive STI testing, even when their chief complaint was not STI related. The ACASI facilitated identification of adolescent ED patients needing STI testing regardless of chief complaint.