Schizophrenic patients, especially those with positive thought disorder, have been shown to show greater facilitation in a semantic priming task than normal controls and depressed patients. While some investigators have replicated these effects, others have not been able to do so. Differences in method and in the chronicity of the patients make it difficult to interpret the discrepancy between findings. The present study was conducted to examine the effect of length of illness (LOI) on semantic priming, a factor that might explain differences in replication success. Thirty schizophrenic patients performed a lexical decision task in a semantic priming paradigm. The LOI of this group ranged from 5 to 31 years. Facilitation in the priming task was positive for patients with short LOIs and declined to negative values in patients with long LOIs. The gradient of decline was significant. Analysis of the separate components of facilitation indicates that the decline was primarily due to a steady reduction of the speed with which patients responded to the associated word pairs; speed of response to non-associated pairs was unaffected. As this was a cross-sectional study it is not possible to determine whether the decline can be attributed to a cumulative effect of length of illness, or to prior initial differences in pathology that affect the probability of chronicity.