A study of inflorescence and flower development in 12 species from four of the six subgenera of Gunnera (Gunneraceae) was carried out. In the species of subgenus Panke, initiation of floral apices along the partial inflorescences is acropetal but ends up in the late formation of a terminal flower, forming a cyme at maturity. The terminal flower is the largest and the most complete in terms of merosity and number of whorls and thus it is the most diagnostic in terms of species-level taxonomy. The lateral flowers undergo a basipetal gradient of organ reduction along the inflorescence, ranging from bisexual flowers (towards the distal region) to functionally (i.e. with staminodia) and structurally female flowers (towards the proximal region). Our results show that the terminal structure in Gunnera is a flower rather than a pseudanthium. The terminal flower is disymmetric, dimerous and bisexual, representing the common bauplan for Gunnera flowers. It has a differentiated perianth with two sepals and two alternate petals, the latter opposite the stamens and carpels. Comparisons with other members of the core eudicots with labile floral construction are addressed. We propose vegetative and floral putative synapomorphies for the sister-group relationship between Gunneraceae and Myrothamnaceae. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 160, 262–283.