Many researchers have assumed that testing for semantic priming in a lexical decision task assesses whether aphasic patients have preserved automatic semantic processing. However, research with normal subjects has shown that priming in lexical decision may result from controlled processing (e.g. Balota & Lorch, 1986). The present research developed a lexical decision task that would reflect automatic priming and tested aphasic patients on this task. Also, the nature of the semantic knowledge revealed through priming was investigated by examining priming for words that are semantically related but share no associates (e.g. poem-novel). Contrary to the findings of Milberg & Blumstein (1981), automatic priming was obtained for a Broca's aphasic. Also, priming was not revealed for semantically related words sharing no association, indicating that automatic priming relies on lexical associations between words, not semantic knowledge. Automatic semantic priming does not appear to be a useful measure for assessing semantic knowledge organization.