Postpartum depression and other perinatal mental illnesses are common complications of childbirth. The majority of research on these conditions has been conducted in heterosexual women; however, increasing numbers of women are choosing to parent in the context of lesbian relationships. Although many of the fundamental aspects of the transition to parenthood are shared between lesbian and heterosexual mothers, lesbian mothers may differ from heterosexual parents on a number of variables that have been previously associated with perinatal mental health. Lesbian mothers may be more likely than their heterosexual peers to lack social support, particularly from their families of origin, and may be exposed to additional stress due to homophobic discrimination. However, the likelihood that lesbian pregnancies will be planned, together with the relatively equal division of child-care labour observed in lesbian couples, may offer protection from perinatal depression. The study of perinatal mental health in lesbian mothers is warranted, both to ensure that the mental health needs of this largely invisible population are being met, and to further illuminate the role of psychosocial stress in perinatal mental health in all women.