The purpose of this study was to develop a theoretical explanation of the process of becoming a father and how this impacted on smoking behaviours of men during their partner's pregnancy. Grounded theory method and constant comparative analysis was used to explain data from interviews with 23 men from rural Midwest USA. Becoming a father prompted participants to seek ways to protect their partners and babies from the effects of cigarette smoking. First-time fathers were particularly receptive to quitting smoking early in their partner's pregnancy, whereas motivation to quit declined as pregnancy progressed for first-time fathers and other fathers. Although protecting their families from cigarette smoke remained constant, men's smoking goals and behaviours became increasingly focused on minimising harm rather than quitting as their partner's pregnancy progressed. Findings support the view that the transition to fatherhood is an opportune time in which to encourage and support smoking behaviour change for male partners. Theoretical insights provide understanding of why and when expectant fathers attempt to quit smoking and reduce harm to their families. Future research should focus on appropriate and timely interventions to maximise the success of men to quit smoking when they are naturally motivated to do so.