BACKGROUND: Despite its importance to sexual health and wellbeing, sexual function is given little attention in sexual health policy. Population-based studies are needed to understand sexual function across the life course. METHODS: We undertook a probability sample survey (the third National Survey of Sexual Attitudes and Lifestyles [Natsal-3]) of 15,162 individuals aged 16-74 years who lived in Britain (England, Scotland, and Wales). Interviews were done between Sept 6, 2010, and Aug 31, 2012. We assessed the distribution of sexual function by use of a novel validated measure (the Natsal-SF), which assessed problems with individual sexual response, sexual function in a relationship context, and self-appraisal of sex life (17 items; 16 items per gender). We assess factors associated with low sexual function (defined as the lowest quintile of distribution of Natsal-SF scores) and the distribution of components of the measure. Participants reporting one or more sexual partner in the past year were given a score on the Natsal-SF (11,690 participants). 4122 of these participants were not in a relationship for all of the past year and we employed the full information maximum likelihood method to handle missing data on four relationship items. FINDINGS: We obtained data for 4913 men and 6777 women for the Natsal-SF. For men and women, low sexual function was associated with increased age, and, after age-adjustment, with depression (adjusted odds ratio 3·70 [95% CI 2·90-4·72] for men and 4·11 [3·36-5·04] for women) and self-reported poor health status (2·63 [1·73-3·98] and 2·41 [1·72-3·39]). Low sexual function was also associated with experiencing the end of a relationship (1·52 [1·18-1·95] and 1·77 [1·44-2·17]), inability to talk easily about sex with a partner (2·36 [1·94-2·88] and 2·82 [2·28-3·48]), and not being happy in the relationship (2·89 [2·32-3·61] and 4·10 [3·39-4·97]). Associations were also noted with engaging in fewer than four sex acts in the past 4 weeks (3·13 [2·58-3·79] and 3·38 [2·80-4·09]), having had same sex partners (2·28 [1·56-3·35] and 1·60 [1·16-2·20]), paying for sex (in men only; 2·62 [1·46-4·71]), and higher numbers of lifetime sexual partners (in women only; 2·12 [1·68-2·67] for ten or more partners). Low sexual function was also associated with negative sexual health outcomes such as experience of non-volitional sex (1·98 [1·14-3·43] and 2·18 [1·79-2·66]) and STI diagnosis (1·50 [1·06-2·11] and 1·83 [1·35-2·47]). Among individuals reporting sex in the past year, problems with sexual response were common (41·6% of men and 51·2% of women reported one or more problem) but self-reported distress about sex lives was much less common (9·9% and 10·9%). For individuals in a sexual relationship for the past year, 23·4% of men and 27·4% of women reported an imbalance in level of interest in sex between partners, and 18·0% of men and 17·1% of women said that their partner had had sexual difficulties. Most participants who did not have sex in the past year were not dissatisfied, distressed, or avoiding sex because of sexual difficulties. INTERPRETATION: Wide variability exists in the distribution of sexual function scores. Low sexual function is associated with negative sexual health outcomes, supporting calls for a greater emphasis on sexual function in sexual health policy and interventions. FUNDING: Grants from the UK Medical Research Council and the Wellcome Trust, with support from the Economic and Social Research Council and the Department of Health.