The Guinea-Bissau Twin Registry was established by the Bandim Health Project (BHP), which maintains a health and demographic surveillance system (HDSS) in Bissau, the capital of Guinea-Bissau. The study aims to describe the twinning rate and mortality patterns among newborn twins, including mortality risk factors and hospitalization patterns, and to focus particularly on risk factors for metabolic disorders. The Guinea-Bissau Twin Registry includes two cohorts: almost 2,500 newborns born between 2009 and 2013 at a local hospital and around 1,000 young people aged 5 to 30 years registered with the Bandim HDSS. Each cohort includes both twins and singleton controls. Baseline assessment occurred shortly after birth for the newborn cohort with follow-up after 2, 6, and 12 months, and afterward at 6-month intervals until 2013. The young cohort underwent baseline assessment from 2009 to 2011 and continues to be regularly followed up through the HDSS.