Many patients with progressive fluent aphasia present with poor verbal comprehension and profound word-finding difficulties in the context of much better picture comprehension and object use. The Japanese term Gogi (literally "word-meaning") aphasia matches this behavioural pattern. The alternative label of semantic dementia is most often used for these patients and this term emphasises a generalised degradation of conceptual knowledge that encompasses both verbal and nonverbal comprehension. The study presented here investigates whether progressive fluent aphasia has a functional impairment limited to the verbal domain (Gogi aphasia) or more widespread involvement of all conceptual knowledge (semantic dementia). We report data collected from a patient with progressive fluent aphasia, IW, who presented with profound word-finding difficulties and relatively poor word comprehension. The predictions of three theoretical interpretations of this pattern are investigated in a series of experimental tasks. We argue that IW's poor verbal comprehension and anomia cannot easily be explained as an impairment to either a semantic lexicon or a modality-specific verbal semantic system. Instead we favour an explanation in terms of a single impairment to a unitary semantic system within a framework that emphasises the underlying differences in the mapping between surface form and meaning, for words and pictures. We demonstrate how IW's pattern of data can be replicated in an implemented connectionist network that includes a systematic mapping for pictures but an arbitrary relationship for words. We conclude that although Gogi aphasia maybe an accurate clinical description of the most striking features observed in progressive fluent aphasia, the disorder is primarily a progressive loss of conceptual knowledge - it is semantic dementia.