While the literature on women's experience of postnatal depression (PND) is expanding, little is known about lay people's beliefs about the disorder. Beliefs about the causes of PND held by women with young children are important: firstly, because they may provide health professionals with insight into the type of support and advice that proliferates in the informal support networks of young mothers; secondly, they may shed light on the decision by many women with PND not to seek professional assistance. In the first study to be reported, 58 members of the general population were asked to respond to an open-ended question: "What do you think causes postnatal depression?" Beliefs mentioned by participants (in addition to those drawn from the literature) were used to construct items in questionnaire subsequently given to 284 women with preschool children. Participants were required to indicate how important they felt each of the items was as a possible cause of PND. Responses were factor analysed in order to determine the underlying structure of causal beliefs. Five factors emerged: stress surrounding childbirth and childcare, social support, social expectations, psychological and biological variables. Beliefs were related to knowing someone with PND, level of participation in the paid workforce, and exposure to PND through professional training. The theoretical and clinical implications of these findings will be discussed.