Background: Selenium, a trace element, is one of the important micronutrients with protective properties against oxidative stress, modulation of fetal metabolism, growth and development, and maintenance of defence against infection. Its deficiency may result in adverse maternal and perinatal outcome including low birth weight. This may occur through reduced antioxidant activities of selenium dependent antioxidant systems like glutathione peroxidase, iodothyronine deionase and thioredoxin reductase, resulting in oxidative damage of human cells, proteins and enzymes during embryonal and fetal periods. Objective: To determine the relationship between maternal serum selenium and birth weight in HIV positive and negative pregnant women. Methods: A cross-sectional study involving HIV positive and HIV negative pregnant women at term was carried out. Women were enrolled at prenatal care clinics of the Lagos University Teaching Hospital (LUTH) at which time anthropometric measurements and sociodemographic data were recorded. Blood samples were also collected for selenium level estimation using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The serum selenium concentration was compared between the two groups. Also, the relationship of maternal selenium concentration and birth weight was determined in both groups. Birth weight was measured at hospital delivery.