Asthma is the most common chronic disease of childhood. Recent evidence has shown a socio-economic gradient in its distribution. This paper examines whether a number of factors argued to have led to a rise in the incidence of asthma might also explain the social gradient. Several of these have been the object of policy intervention, though not necessarily with the aim of lowering childhood respiratory conditions. Using a large cohort study (the Avon Longitudinal Study of Parents and Children) we find significant inequalities in three respiratory conditions in middle childhood. We investigate eight potential mediating factors: exposure to other children in infancy, child's diet, poor housing conditions, maternal smoking, parental history of asthma, poor child health at birth, maternal age at child's birth and local deprivation. We find that each of these alone typically explains a relatively modest part of each respiratory inequality, with child's diet, local deprivation and maternal smoking generally the most important. But taken together, the mediating factors account for a substantial part of the respiratory inequalities. So the socio-economic gradient appears to operate through a number of inter-correlated pathways, some of which may be amenable to policy intervention.