The genus Hydrangea (Hydrangeaceae) is with more than 1,000 cultivars and hybrids already since centuries a very popular ornamental plant. These ornamentals with inflorescences with attractive marginal flowers are commonly known as "hortensias" and descend from Asian species such as H. macrophylla and H. aspera. The Asian representatives of Hydrangea have been relatively well studied although the species boundaries and relationships continue to be disputed. However, the nearly exclusively American section Cornidia consisting of approximately 15 accepted lianescent species and according to our ongoing studies a surprisingly high number of species new to science is very poorly known and nearly absent from living collections. In the framework of our ongoing Red List project in cooperation with BGCI, the IUCN/SSC Global Tree Specialist Group and the Global Trees Campaign (see http://www.bgci.org/ourwork/Hydrangeas), we are assessing the status of the species of Hydrangea s.l. in America and Asia. In Mexico, this group seems not only to be undercollected but also highly endangered because of its occurrence in hyper-conserved primary forests, which are becoming rarer every day. Our recent exploring field work in central and southeast Mexico (January-February, and July-August 2011) revealed the presence of several new species which had never been noticed and collected before and which are already at risk. The fact that these remarkable plants remained undiscovered up to now is especially striking because of their growth form and size. The largest individuals grow more than 30 meters high, with a thick flexible stem which thickens to the very top where it starts branching like a shrub and flowers overwhelmingly above the tree canopy. Because of their size, material for further study can only be collected using mountaineering equipment, a technique which is rarely applied in botanical studies. This recent work urges the need for 1) further exploration in central and southern Mexico for additional localities of these new species, 2) characterization of their habitat, 3) description of these new species, including detailed morphological-taxonomical work and molecular analyses studying their relationships and genetic diversity, 4) evaluation of their status applying the IUCN Red List categories and criteria, and 5) in situ and ex situ conservation of these species in cooperation with CONANP (Comisión Nacional de Áreas Naturales Protegidas) and the local indigenous people living near the localities.