Speech samples were obtained that were analyzed for voice onset time (VOT) for 40 nondemented English speaking subjects, 20 with mild and 20 with moderate Parkinson's disease. Syntax comprehension and cognitive tests were administered to these subjects in the same test sessions. VOT disruptions for stop consonants in syllable initial position, similar to those noted for Broca's aphasia, occurred for nine subjects. Longer response times and errors in the comprehension of syntax as measured by the Rhode Island Test of Sentence Comprehension (RITLS) also occurred for these subjects. Anovas indicate that the VOT overlap subjects had significantly higher syntax error rates and longer response times on the RITLS than the VOT nonoverlap subjects--F(1, 70) = 12.38, p less than 0.0008; F(1, 70) = 7.70, p less than 0.007, respectively. The correlation between the number of VOT timing errors and the number of syntax errors was significant. (r = 0.6473, p less than 0.01). VOT overlap subjects also had significantly higher error rates in cognitive tasks involving abstraction and the ability to maintain a mental set. Prefrontal cortex, acting through subcortical basal ganglia pathways, is a component of the neural substrate that regulates human speech production, syntactic ability, and certain aspects of cognition. The deterioration of these subcortical pathways may explain similar phenomena in Broca's aphasia. Results are discussed in relation to "modular" theories.