The oligarchy hypothesis proposes that large areas of Amazonian plant communities are dominated by limited sets of species. We tested this hypothesis by (1) quantifying dominance of the 10 most common species genera and families in each region; and (2) assessing the consistency of relative abundance ranks between areas and across scales in dominance patterns for trees and lianas in two distant Amazonian regions ([similar]1900 km) the Yasunv? and Madidi National Parks in Ecuador and Bolivia respectively. The analyses were based on sixty-nine 0.1-ha plots in which all woody plants with a diameter at breast height (dbh) [greater-than-or-equal]2.5 cm were inventoried (19 775 individuals and 1729 species in total). The plots were located at two Yasunv? and five Madidi sites with an average of 10 plots per site. Overall oligarchic dominance was pronounced at all the spatial scales investigated although decreasing with increasing scale. Cross-scale relative abundance ranks were more consistent in Yasunv? than in Madidi while no such difference was apparent within single sites. Quantitative dominance and consistency of relative abundance ranks increased with taxonomic rank being stronger at the family level than at genus and species levels. Species-level dominance was somewhat stronger within the 10 most common families in either region than in other families. Dominance was similarly strong for canopy (dbh [greater-than-or-equal]10 cm) and understorey trees (dbh <10 cm) and less pronounced among lianas. In conclusion our results provide strong evidence that western Amazonian forests can be dominated by limited oligarchies of species genera and families over large expanses.