OBJECTIVES: To assess the effectiveness of an internet-accessed STI (e-STI) testing and results service on testing uptake among young adults (16-30 years) who have never tested for STIs in London, England. METHODS: We conducted secondary analyses on data from a randomised controlled trial. In the trial, participants were randomly allocated to receive a text message with the web link of an e-STI testing and results service (intervention group) or a text message with the link of a website listing the locations, contact details and websites of seven local sexual health clinics (control group). We analysed a subsample of 528 trial participants who reported never testing for STIs at baseline. Outcomes were self-reported STI testing at 6 weeks, verified by patient record checks, and time from randomisation to completion of an STI test. RESULTS: Uptake of STI testing among 'never testers' almost doubled. At 6 weeks, 45.3% of the intervention completed at least one test (chlamydia, gonorrhoea, syphilis and HIV), compared with 24.1% of the control (relative risk [RR] 1.88, 95% CI 1.47 to 2.40, p<0.001). For chlamydia and gonorrhoea testing combined, uptake was 44.3% in the intervention versus 24.1% in controls (RR 1.84, 95% CI 1.44 to 2.36, p<0.001). The intervention reduced time to any STI test (restricted mean survival time: 29.0 days vs 36.3 days, p<0.001) at a time horizon of 42 days. CONCLUSIONS : e-STI testing increased uptake of STI testing and reduced time to test among a young population of 'never testers' recruited in community settings. Although encouraging, questions remain on how best to manage the additional demand generated by e-STI testing in a challenging funding environment. Larger studies are required to assess the effects later in the cascade of care, including STI diagnoses and cases treated.