We examined four patients with transcortical sensory aphasia and eight with milder language disturbances but with similar thalamic and/or temporo-occipital lesions. Specific attention was paid to differentiation of the computed tomographic lesion site of the milder cases from the transcortical sensory aphasia cases. The critical lesion for transcortical sensory aphasia in these patients involved pathways in the posterior periventricular white matter adjacent to the posterior temporal isthmus, pathways that are probably converging on the inferolateral temporo-occipital cortex. Analysis of the language function of these patients, of the influence of sensory modalities on language function, and of the interaction between semantic memory and semantic lexical functions suggests the existence of a specific brain system for semantic functions. This semantic system has a particular distributed anatomy. We propose that damage to this system may have a variety of clinical manifestations in language and in memory, depending on the exact lesion configuration.