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The comprehension of 100 different spoken English sentences was studied in 39 patients with mild to moderate Parkinson's disease (PD) using the Rhode Island Test of Language Structure. Eight of the PD patients were mildly demented, and the remaining 31 showed no cognitive deficits. Seven of the eight mildly demented patients had high comprehension error rates for sentences that had moderately complex syntax. The error rates of 28 of the 31 other, nondemented patients were low, similar to those of normal control subjects. The difference between the error rates of the demented and nondemented patients was significant (p<.02, two-tailed t-test). The data argue against some current “modular” theories that segregate linguistic from other cognitive behavior. The destruction by PD of the midbrain regions that stimulate the frontal cortex may be responsible for both cognitive and syntactic comprehension deficits.