The aim of this study was to establish which behaviors were considered to constitute sexual relations and to compare a group of undergraduates in the UK with a group in the US. An opportunistic sample of 190 female and 124 male UK undergraduate university students was surveyed by questionnaire. The main outcome measure was percentage of responses to 11 different behaviors believed to constitute "having sex." The majority of respondents regarded having sex as involving penile-vaginal and penile-anal intercourse. One-third of respondents regarded oral-genital contact as having sex, around 17% regarded touching genitals, whilst 6% regarded oral or other touching of breasts and nipples as constituting having sex. There were significant gender- and age-related differences in responses. These findings broadly support the findings of an earlier US study. It is clear that British students hold divergent opinions about which behaviors do and do not constitute having sex. The age-related trends merit further exploration. Any studies of sex-related behaviors need to specify precisely which are encompassed by the terms used.