The various ways in which infants may be contaminated with HIV are described. Transmission by blood perfusion persists in only some countries where HIV detection in blood products is not performed and sterilization is doubtful. Transplacental contamination is well documented by the finding of HIV in various foetal tissues, amniotic fluid, placenta and foetal blood; it seems to be predominant. Transmission during delivery and breast-feeding plays a lesser role, but it cannot be excluded. Horizontal transmission is extremely rare. The frequency of vertical transmission varies from one series to another, but the most often quoted figure is approximately 30 p. 100. An Atlanta C.D.C. stage III or IV and a low percentage of T4 lymphocytes are thought to facilitate the mother-to-foetus transmission. Many elements are still imperfectly known; their elucidation requires further studies on large series of subjects.