[en] Sixty years after first investigations on producing uranium from coal ash, this uranium source of supply has regained a strong interest. While the world consumption of coal keeps rising, several papers tackle radiological health issues. Besides, uranium-bearing coal deposits are sometimes mentioned as a potential source of supply for the nuclear fuel. While uranium production from coal ash has remained sub-economic for decades, the emergence of new projects begs the questions again. How much coal ash do we have? Are the coal deposits all rich in uranium? Can we produce them all? The present study gives an estimation of both the world resources and the production capacities of uranium as a by-product of coal. It shows that there are significant quantities of technically accessible uranium in the world coal reserves. Yet, most of these quantities correspond to very low-grade ores. Potential reserves should be less than 200 ktU. In terms of production capacities, a realistic potential should not exceed 700 tU/year, that is approximately 1% of current needs. (author)