Sex Transm Dis — New York: Harrington Park Press. Am J Public Health — We apply our estimates to ONS census population estimates which coincide with midway through Natsal-3 fieldwork of men and women aged 16—74 years in Britain to quantify the size of sexual minority populations according to the different dimensions of sexual orientation. The other authors declare that they have no competing interests. Among women, reporting of same-sex experience without any genital contact and same-sex sex ever and in the past 5 years declined with age. Sexually Transmitted Infections 84— Thus differences in the primary purpose of the surveys and therefore the context in which these sensitive questions are being asked, are likely partly responsible for variation in estimates.