BACKGROUND: Excessive use of alcohol is associated with poor sexual health, but the clinical effectiveness and cost-effectiveness of brief alcohol intervention in this setting has not been investigated. OBJECTIVE: To examine the effects and cost-effectiveness of brief intervention for excessive alcohol consumption among people who attend sexual health clinics. DESIGN: A two-arm, parallel-group, single-blind, pragmatic, randomised controlled trial. Participants were randomised via an independent and remote telephone randomisation service using permuted blocks, stratified by clinic. SETTING: Study participants were recruited from three sexual health clinics in central and west London. PARTICIPANTS: For inclusion, potential participants had to be aged ≥ 19 years, drink excessive alcohol according to the Modified-Single Alcohol Screening Question, and be willing to provide written informed consent. We excluded those who were unable to communicate in English sufficiently well to complete the baseline assessment and those who could not provide contact details for the follow-up assessment. INTERVENTIONS: Brief advice was delivered by the treating clinician and comprised feedback on the possible health consequences of excessive drinking, a discussion of whether the participant's clinic attendance was linked to current alcohol use, written information on alcohol and health and an offer of an appointment with an alcohol health worker (AHW). Appointments with AHWs took place either in person or by telephone, lasted up to 30 minutes, and used the 'FRAMES' (Feedback about the adverse effects of alcohol, an emphasis on personal Responsibility for changing drinking behaviour, Advice about alcohol consumption, a Menu of options for further help and advice, an Empathic stance towards the patient and an emphasis on Self-efficacy) approach. Those in the control arm of the trial were offered a copy of a leaflet providing general information on health and lifestyle. MAIN OUTCOME MEASURES: Outcomes were assessed 6 months after randomisation. The primary outcome was mean weekly alcohol consumption during the previous 90 days. The main secondary outcome was unprotected sex during this period. RESULTS: Eight hundred and two people were recruited to the study of whom 592 (74%) were followed up 6 months later. Among 402 participants who were randomised to brief intervention, 397 (99%) received brief advice from the treating clinician and 81 (20%) also received input from an AHW. The adjusted mean difference in alcohol consumption after 6 months was -2.33 units per week [95% confidence interval (CI) -4.69 to 0.03 units per week, p = 0.053] for those in the active arm compared with the control arm. Unprotected sex was reported by 154 (53%) of those who received brief intervention and by 178 (59%) of controls (adjusted odds ratio 0.89, 95% CI 0.63 to 1.25, p = 0.496). Participants randomised to brief intervention reported drinking a mean of 10.4 units of alcohol per drinking day compared with 9.3 units among control participants (difference 1.10, 95% CI 0.29 to 1.96, p = 0.009). We found no statistically significant differences in other outcomes. Brief intervention (brief advice and input from an AHW) cost on average £12.60 per person to deliver and did not appear to provide a cost-effective use of resources. CONCLUSIONS: Introduction of universal screening and brief intervention for excessive alcohol use among people who attend sexual health clinics does not result in clinically important reductions in alcohol consumption or provide a cost-effective use of resources. While people attending sexual health clinics may want to achieve better sexual health, attempts to reduce alcohol consumption may not be seen by them as a necessary means of trying to achieve this aim. TRIAL REGISTRATION: This trial is registered as ISRCTN 99963322. FUNDING: This project was funded by the NIHR Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 18, No. 30. See the NIHR Journals Library website for further project information.