Vitamin D is a well-known immunomodulator. The relationship between vitamin D status and human immunodeficiency virus (HIV) infection has attracted attention in recent literature. Evidence suggests there may be increased prevalence of vitamin D deficiency in HIV-infected children compared with HIV sero-negative counterparts. One study has linked increased mother-to-child transmission of HIV, child mortality and adverse perinatal outcomes to vitamin D deficiency. This article provides a brief introduction into the emerging information surrounding the role of vitamin D and the placenta on mother-to-child transmission of HIV.