Abstract: This work analyzes the impact of the concentration level within the natural resource sector in the relationship between resource abundance and economic growth, with which we hope to be able to explain why natural resources are a “blessing” for some countries but a “curse” in others. An extensive dataset with highly disaggregated export data is built for the period 1971-2000, and the model is estimated using the System-GMM approach to avoid the problems induced by non-observable country-specific effects and the joint endogenity of the explanatory variables. Then, the effect of resource abundance on economic growth is analyzed according to the level of within-sector concentration, obtaining the marginal effects using the delta method. The empirical results show that the presence of a “curse” is conditional on how concentrated the primary sector is. The disaggregated analysis for distinct kinds of natural resources shows that the effect is driven by minerals and ores, while the relationship for the other kinds of resources is less clear. Finally, although the result of a “curse” that is conditional on within-sector concentration for the minerals and ores holds despite the institutional quality of each country, the order of magnitude of the effect does depend on institutional quality.