This paper explores the ways in which women's sterilisation decisions are influenced by the combination of a preference for male children and a desire for smaller family size among young married women in two urban slums in Bengaluru, India. While both son preference and an emphasis on sterilisation are well-known demographic characteristics of most South Asian countries, relatively little research has been conducted that links the two. We take advantage of a longitudinal survey of 416 unsterilised married women aged 16-25 to explore how having sons and the number of children influence a woman's sterilisation decision. Discrete-time event history techniques are used to estimate two models: the first examines the effect of having sons and number of children separately, and the second examines them in combination in the form of an interaction. The results suggest sterilisation is motivated by son preference mainly at lower parities (three or fewer children) and by concerns about family size at higher parities. Understanding how sterilisation and other reproductive behaviours are influenced by the interaction of family size and sex preferences will help policy-makers and programmers to meet the needs of women while continuing to address discriminatory behaviour against females.