Fathers work less after having a first born son rather than a daughter in the UK in the short-term. Fathers are on average 4.5 percentage points less likely to be in employment. On the intensive margin, fathers work on average 2.2 fewer hours per week in the first year of the child’s life. We exploit the randomness of the gender of the first child to identify these effects, and use the UK Household Longitudinal Study. The effect for degree educated fathers is consistent with an explanation that fathers take time off work to increase parental inputs received by sons, given that their partners also work less after a first born son rather than a daughter. The effect for non-degree educated fathers is consistent with an explanation that fathers take time off work to reduce the burden of childcare on their partners, given that their partners work more after a first born son rather than a daughter. We show that child birthweight, pure son preference or the higher costs of raising daughters than sons do not drive the effects.