Research on concurrent sexual partnerships is hindered by lack of accurate partnership data. Using unique life-history calendar data from a population-based sample of youths aged 18-24 in urban Kenya, we estimated the prevalence and correlates of concurrency. In the sixth month before the survey, 3.5 per cent of females and 4.0 per cent of males were engaged in concurrent sexual partnerships. In the previous 9.5 years, males experienced more concurrent partnerships than females and they were of shorter duration. Using survival analysis, we find that the characteristics of initial partnerships affect entry into a second (concurrent) relationship. For females, geographic separation from a partner increases the risk of concurrency, while concurrency is positively associated with the duration of the initial relationship for males. For both sexes, the perception of partner infidelity increases the risk, suggesting that concurrency expands individuals' sexual networks and bridges additional networks involving partners' other sexual partners.