A left parietal infarct in a prelingually deaf person resulted in an aphasia for both American Sign Language (ASL) and written and finger-spelled English. Originally the patient had a nearly global aphasia affecting all language systems. By five to seven weeks post-onset her symptoms resembled those of hearing aphasics with posterior lesions: fluent but paraphasic signing, anomia, impaired comprehension and repetition, alexia, and agraphia with elements of neologistic jargon. In addition, there was a pronounced sequential movement copying disorder, reduced short-term verbal memory and acalculia. In general, the patient's sign errors showed a consistent disruption in the structure of ASL signs which parallels the speech errors of oral aphasic patients. We conclude that most aphasic symptoms are not modality-dependent, but rather reflect a disruption of linguistic processes common to all human languages. This case confirms the importance of the left hemisphere in the processing of sign language. Furthermore, the results indicate that the left supramarginal and angular gyri are necessary substrates for the comprehension of visual/gestural languages.