BACKGROUND: Physical and mental health could greatly affect sexual activity and fulfilment, but the nature of associations at a population level is poorly understood. We used data from the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3) to explore associations between health and sexual lifestyles in Britain (England, Scotland, and Wales). METHODS: Men and women aged 16-74 years who were resident in households in Britain were interviewed between Sept 6, 2010, and Aug 31, 2012. Participants completed the survey in their own homes through computer-assisted face-to-face interviews and self-interview. We analysed data for self-reported health status, chronic conditions, and sexual lifestyles, weighted to account for unequal selection probabilities and non-response to correct for differences in sex, age group, and region according to 2011 Census figures. FINDINGS: Interviews were done with 15,162 participants (6293 men, 8869 women). The proportion reporting recent sexual activity (one or more occasion of vaginal, oral, or anal sex with a partner of the opposite sex, or oral or anal sex or genital contact with a partner of the same sex in the past 4 weeks) decreased with age after the age of 45 years in men and after the age of 35 years in women, while the proportion in poorer health categories increased with age. Recent sexual activity was less common in participants reporting bad or very bad health than in those reporting very good health (men: 35·7% [95% CI 28·6-43·5] vs 74·8% [72·7-76·7]; women: 34·0% [28·6-39·9] vs 67·4% [65·4-69·3]), and this association remained after adjusting for age and relationship status (men: adjusted odds ratio [AOR] 0·29 [95% CI 0·19-0·44]; women: 0·43 [0·31-0·61]). Sexual satisfaction generally decreased with age, and was significantly lower in those reporting bad or very bad health than in those reporting very good health (men: 45·4% [38·4-52·7] vs 69·5% [67·3-71·6], AOR 0·51 [0·36-0·72]; women: 48·6% [42·9-54·3] vs 65·6% [63·6-67·4], AOR 0·69 [0·53-0·91]). In both sexes, reduced sexual activity and reduced satisfaction were associated with limiting disability and depressive symptoms, and reduced sexual activity was associated with chronic airways disease and difficulty walking up the stairs because of a health problem. 16·6% (95% CI 15·4-17·7) of men and 17·2% (16·3-18·2) of women reported that their health had affected their sex life in the past year, increasing to about 60% in those reporting bad or very bad health. 23·5% (20·3-26·9) of men and 18·4% (16·0-20·9) of women who reported that their health affected their sex life reported that they had sought clinical help (>80% from general practitioners; <10% from specialist services). INTERPRETATION: Poor health is independently associated with decreased sexual activity and satisfaction at all ages in Britain. Many people in poor health report an effect on their sex life, but few seek clinical help. Sexual lifestyle advice should be a component of holistic health care for patients with chronic ill health. FUNDING: Grants from the UK Medical Research Council and the Wellcome Trust, with support from the Economic and Social Research Council and Department of Health.