The nature of semantic memory deficit in Alzheimer's disease is still a matter of controversy. To clarify this issue, we examined the evolution of semantic memory impairment in 24 Alzheimer's disease patients by means of a longitudinal study. We used two semantic tasks, one explicit and the other implicit, to evaluate the integrity of the same concepts. The explicit task was a semantic knowledge task composed of naming and questions, involving superordinate and attribute knowledge of concepts. The implicit task, a lexical decision task, assessed semantic priming and allowed a very pure measurement of semantic memory. In this task, related pairs of words had coordinate (e.g. "tiger-lion") or attribute ("tiger-stripe") relationships. In the coordinate relation between two words, the semantic priming performances were at first paradoxical: they increased abnormally (hyperpriming) before falling down, whereas in the attribute condition, the priming effects were first normal and then started to decrease in the final sessions (hypopriming). Compared with the semantic knowledge performance, these apparently disconcerting results reflect a coherent pattern of semantic memory degradation in Alzheimer's disease that is a progressive deterioration starting with specific attribute information. The data reveal in an astonishing yet striking manner the dynamic semantic memory degradation in Alzheimer's disease through the apparently paradoxical semantic priming effects.