This study examines the factors that influence sexual behaviours among young unmarried males living in urban slums in India, a population characterised by high vulnerability to STI and HIV infections. Data were collected from 600 unmarried male slum youth to examine the relationships between three outcomes of heterosexual behaviour (age at sexual initiation, number of sexual partners and condom use at first intercourse) and factors within the family/household social environment, peer environment and individual level during the respondents' formative years. Regression analyses found factors within the family and peer-level domains such as growing up in a two-parent household, having adequate levels of parental supervision, experiencing violence in the home and having peers who drank significantly impact the age of sexual debut and the subsequent number of lifetime partners. Condom use at first intercourse appears to be significantly influenced by age of sexual debut and type of sexual partner. These findings indicate that in low-resource urban settings, the influence of family and friends as role models can play an important role in determining the initiation of sexual behaviour among male youth and in making behaviour choices that result in favourable health outcomes.