The main objective of this study was to evaluate the effectiveness of a school-based preventive oral health programme to reduce the dmft (decay, missing and filled teeth for deciduous dentition) and DMFT (Decay, Missing, Filling teeth for definitive dentition) index among schoolchildren from a rural area of western Africa – Uno’s Island in Guinea Bissau. A preventive school-based program was implemented in a primary school in Guinea-Bissau between 2013 and 2016. The children received educational lectures, toothbrush instruction and topical fluor every 6 months. Teachers also received instructions regarding the care of the oral hygiene. Once per year they were given 1450 ppm toothpaste and a toothbrush per child and all children were screened, determining their dmft/DMFT index at baseline (2013) and three years after (2016). At baseline (2013) the total mean (SD) dmft scores were 0.76 (1.30) and the total mean (SD) DMFT scores were 0.56 (1.06). In year 3 (2016) the total mean (SD) dmft scores were 0.33 (0.99) and the total mean (SD) DMFT scores were 0.15 (0.57). The study aims to demonstrate that prevention programs benefit the oral and overall health of children, by showing a decrease of dmft/DMFT index.