Patients with idiopathic Parkinson's disease (PD) were asked to identify the agent of the action in orally presented sentences with subject-relative or object-relative center-embedded clauses while simultaneously performing a secondary task that was less resource-demanding (finger tapping) or more resource-demanding (recognition span). We found that a subgroup of PD patients with impaired sentence comprehension at baseline (no secondary task) did not differ from random in their accuracy understanding all types of sentences during the more demanding (recognition span) condition and also had difficulty understanding the most complex sentences during the less demanding (finger tapping) condition. Control subjects and PD patients without baseline sentence comprehension difficulty were random only in their comprehension of the most complex sentences under the more demanding (recognition span) secondary task condition. Examination of response latencies for accurately understood sentences revealed only an effect for the type of sentence, and this was equally evident across all groups of subjects and regardless of the condition under which the sentences were administered. The sensitivity of PD patients' sentence comprehension accuracy to secondary task resource demands is most consistent with the hypothesis that limited cognitive resources contribute to sentence comprehension difficulty in PD.