The etiology of sleeping sickness was unknown until the early twentieth century. This African disease soon became the main obstacle to European colonization. Sending scientific missions to the colonies to monitor its progression in loco thus became inevitable. Portugal sent the first research mission to Angola in 1901, and the Royal Society of London sponsored two British missions to study the disease in Entebbe (1902 and 1903). Their results led to a controversy in which Portugal was involved from 1898 to 1904, on the national and international circuits, analysed in this article.