We surveyed church-going youths in Nairobi, Kenya, to investigate denominational differences in their sexual behaviour and to identify factors related to those differences. In comparison with youths attending mainline churches, the youths surveyed at Pentecostal/evangelical churches were less likely to have ever had sex. Furthermore, although male youths in the mainline churches were more likely than their female counterparts to have ever had sex, no such difference emerged between the male and female youths attending Pentecostal/evangelical churches. For youths from both types of churches, not only individual religious commitment (being ‘born again’) but also contextual religiosity (i.e. the extent of socialisation in their faith communities) explained the variations in their sexual behaviour and attitudes. Finally, the effect of denomination on one's intention to have sex in the next 12 months was mediated by the frequency of talk about spiritual issues with church confidants.