Using parsimony and Bayesian analyses, we estimated higher-level relationships within Orchidaceae, focusing on subfamilies and tribes. DNA sequences of part of the low-copy nuclear protein gene Xdh were obtained for 154 taxa including 126 genera of Orchidaceae and outgroup families of Asparagales. The general topology of the Xdh trees is congruent with those published previously based on plastid protein-coding genes and non-coding nuclear ribosomal DNA. The five subfamilies previously recognized are monophyletic and well supported. The results indicate that monandrous condition evolved independently in Vanilloideae and Epidendroideae/Orchidoideae. The analysis clarifies relationships between tribes of Epidendroideae such as Vandeae sensu lato to Collabieae, Epidendreae to Calypsoeae and Malaxideae to Dendrobieae. Also relationships of Bromheadia, Imerinaea, Sirhookera, and achlorophyllous species of Corallorhiza, Gastrodia, Limodorum, Neottia, Wullschlaegelia are for the first time evaluated in a broad molecular phylogenetic framework. Copyright 2010 Elsevier Inc. All rights reserved.