Background: The absolute risk of sexual HIV transmission on stable ART (HIV RNA viral load (VL) <200 c/mL) from condomless sex is unknown. Current limited data are largely focusing on vaginal sex Methodology: The international, observational multi-centre PARTNER study prospectively follows serodifferent couples (heterosexual (HT) and MSM) who had condomless penetrative anal or vaginal sex in the month prior to study entry, and where the HIV+ve partner is on ART. Every 6 months, each partner completes a sexual behaviour questionnaire and the negative partner tests for HIV. Eligibility of follow-up time in this transmission rate analysis required: continued condomless sex; not using PEP or PrEP; and latest VL <200 c/mL. For new diagnoses, phylogenetic analysis compared HIV-1 pol and env sequences by couple, after samples were anonymised. This planned analysis reports the rate of occurrence of linked transmissions. Results: By 1st November 2013, 1110 couples were enrolled. Of 1151 couple-years of follow-up (CYFU), 894 were eligible (586 in HT and 308 in MSM). At baseline, the median duration on ART was 4.9 years (IQR: 1.9-11.4) and couples reported having condomless sex for a median 2 years (IQR: 0.5-6.3). Condomless sex with a different partner outside the partnership during follow-up was reported by 27% MSM and 2% HT HIV-negative partners. During follow-up, couples had condomless sex a median of 45 times/ year (IQR: 16-90). Although some negative partners became HIV positive during FU, no phylogenetically linked transmissions occurred, giving a rate of within-couple HIV transmission during eligible couple-years of zero (95% CI: 0-0.40/100 CYFU)(Table). The upper limit of the 95% CI for the rate of transmission was 0.96/100 CYFU for condomless anal sex (HT and MSM) and 1.97/100 CYFU for condomless receptive anal sex with or without ejaculation (MSM). Conclusions: The overall risk of HIV transmission (in the context of previous sex without transmission) through condomless anal or vaginal sex from HIV positive people on ART with plasma VL < 200 copies/mL is extremely low, but uncertainty over the risk remains, particularly over receptive anal sex. Additional follow-up in MSM is essential to provide more precise estimates for transmission risk given the current assumptions of safety in some communities.