The sex of surviving children is an important determinant of reproductive behaviour in South Asia in general and Pakistan in particular. This cohort study evaluates the role of the sex of children on reproductive intentions and subsequent behaviour of women in urban slums of Karachi, Pakistan. The analysis is based on two rounds of surveys conducted in 1990-91 and 1995 of a cohort of married women aged 15-49 years. The results show that pregnancies became increasingly unwanted as the number of surviving sons increased. The sex of surviving children was strongly correlated with subsequent fertility and contraceptive behaviour. However, rather than an exclusive son preference, couples strove for one or more sons and at least one surviving daughter. The policy implications of the link between overt son preference and low status of women are discussed.