outcome). Methods We did a cross-over cluster-randomised controlled trial comparing APT, as an additional PN method to enhanced patient referral (2018–2019, ISRCTN Reference 15996256). Clusters were 17 UK clinics, assigned in the first period by random permutation. Each period lasted 6 months, with a two-week washout. Participants were heterosexual women and men, 16 years with a positive C. trachomatis test and/or clinical PID, cervicitis, non-gonococcal urethritis or epididymoorchitis, reporting 1 contactable sexual partner in the past six months. Analysis was by intention-to-treat, fitting random effects logistic regression models. Results All clinics completed both periods. 1536 and 1724 index patients provided data in intervention and control phases. In total, 4807 sex partners were reported, of whom 1636 (34%) were committed/steady partners. Characteristics of index cases and partners were balanced. Overall, 293/1536 (19.1%) of index patients chose APT for a total of 305 partners, of whom 248 accepted. In intervention and control phases, 666 (43%) and 800 (46%) of index patients were tested for C. trachomatis; 31 (4.7%) and 53 (6.6%) were positive, adjusted odds ratio (aOR) 0.66 (95% CI 0.41–1.04, p=0.07). The proportion with 1 sex partner treated was 775 (92.7%) in intervention and 760 (89.3%) in the control phase, aOR 1.43 (95% CI 0.98–2.07, p=0.06). There were no significant harms reported. Conclusion The findings suggest a benefit from the offer of APT supplementing enhanced PN. Differences in the outcomes were however small, with lower than expected uptake.