Syntactic trees, or phrase markers, have originally been suggested as a representation of syntax in the mind based on purely linguistic grounds. In this paper, the psychological reality of syntactic trees and hierarchical ordering is explored from another perspective -- that of the neuropsychology of language breakdown. The study reported here examined several syntactic domains that rely on different nodes in the tree – tense and agreement verb inflection, subordinations, interrogatives, and verb movement - through a study of 14 Hebrew and Palestinian Arabic-speaking agrammatic aphasics, and perusal of the cross-linguistic literature. The results show that the impairment in agrammatic production is highly selective, and lends itself to characterization in terms of a deficit in the syntactic tree. The complex pattern of dissociations follows from one underlying deficit - the inaccessibility of high nodes of the syntactic tree to agrammatic speakers. Structures that relate to high nodes of the tree are impaired, while "lower" structures are spared. These findings from neurolinguistics also bear on theoretical questions in linguistics such as the psychological reality of linguistic constructs, split inflection and relative order of functional categories.