In poliomyelitis the risk is of neurological accidents resulting either from the circulation of wild viruses (spontaneous risk) or from the introduction of virus-vaccine (vaccine risk). The spontaneous risk varies both according to the virus type and according to various factors which determine host resistance. Well known among these are: the intrinsic characteristics of the population at a given period (genetic factors and previous experiences of this population with poliomyelitis viruses); environmental factors (water supply, living conditions, rapid urbanization...) which are extremely variable among geographic zones, and cultural factors just as variable among populations. The vaccine risk involves two aspects: the risk of neurological accidents due to use of live vaccine, which has changed from the beginning of vaccination and risk of inefficacy due either to the vaccine itself, whether live or inactived or to temporary ill-understood unfitness of a vaccine to provoke an immunitary response. At the present time, spontaneous and vaccine risks are balanced only in the so called developed countries in which the vaccine risk is accepted as necessary to maintain the spontaneous risk at the lowest level.