A cladistic analysis of the subfamily Aristolochioideae (Aristolochiaceae) based on morphological characters is presented. The analysis includes 65 taxa within the ingroup, which represent all the tribes, subtribes, genera, and infrageneric taxa formally described within the subfamily Aristolochioideae sensu Schmidt. The analysis shows that Aristolochia s.l. is paraphyletic and that Euglypha and Holostylis are not different lineages from Aristolochia. Two of the three subgenera recognized within Aristolochia (Siphisia and Pararistolochia) are shown to be monophyletic. The third subgenus (Aristolochia) is paraphyletic. Most taxa at lower rank levels (sections, subsections, series, and subseries) are shown to be either polyphyletic or paraphyletic. The characters traditionally used for the recognition of these taxa are evaluated and new characters are introduced. Finally, a revised system of classification of the Aristolochioideae based on monophyletic groupings and consisting of two tribes, two subtribes, and five genera is proposed here.