In this paper we present estimates of regional GDP per capita for certain benchmark years during the first half of the 20th century using the method proposed by Geary and Stark (2002). After testing the robustness of the Geary and Stark methodology for the Belgian situation, these estimates are linked to the official regional GDP figures, available since 1955, so that we can cover the whole 20th century. Next we test the contrast put forward by many historians between a ‘poor Flanders’ and a relatively ‘prosperous Wallonia’ around 1900. For the remainder of the analysis Belgium’s nine provinces are used as a geographical unit to take a broader view than just the Flanders/Wallonia controversy. It shows a dramatic reversal of fortune between the northern and southern provinces. Finally, it is investigated whether the 20th century witnessed a process of convergence.