The question whether migration will be central demographic issue in the future of Serbia is assessed through probabilistic population projection. Therefore, the problem on formulating international migration hypothesis, specifically estimation of uncertainty attached to future migration flow, is highlighted. For that purpose, experiences from probabilistic projections of the countries that undergone substantial changes of international migration after joined EU were used. Forecast results up to 2050 shows that Serbia will face significant population ageing with no chances to revert the process during the projection horizon. With respect to decreasing trend of population size, fertility increase is one of two indispensable conditions if Serbia wants to restore its current size. The other is huge immigration that could be accomplished if the country experience fundamental political and economic changes in the forthcoming period. Even then, there is only 15 percent probability that Serbia's population size in 2050 will be higher than it is today. However, if actual emigration pattern of international migration turns into immigration one during the next decades, the uncertainty attached to forecasted indicators of population ageing would be considerably decreased.