Adolescent motherhood tends to occur among vulnerable backgrounds, with increased environmental stressors which might jeopardize the quality of mother-infant interaction. One important factor affecting mother-infant interactions is mothers' depressive symptoms. However, dispositional (e.g. attachment orientation) as well as external characteristics (e.g. social support) might affect this relation. This study examined longitudinally the effects of postpartum depression on maternal behaviours and explored the role played by social support and attachment orientation on this link in a sample of 35 low-income Brazilian adolescent mothers of infants 12 months old. Self-reported maternal depression and social support scales were answered by mothers and maternal behaviours were videotaped and coded at 6 and 12 months postpartum. Self-reported attachment was obtained at 12 months postpartum. Perceptions of social support mediated the association between mothers' depression and mothers' positive behaviours only for those with higher levels of attachment anxiety. Findings could be helpful to intervention programmes targeting adolescent mothers to promote the quality of interaction.