Whereas the consequences of Parkinson's disease (PD) for the performance of single-limb movements are well documented (i.e., bradykinesia, akinesia, rigidity, and tremor), fairly little is known about its implications for the coordination between limb movements. To help resolve this situation an experiment was conducted in which 11 PD patients and 11 control subjects performed rhythmic forearm movements at a comfortable amplitude in the in-phase, antiphase, and single-arm mode at pacing frequencies ranging from 0.5 to 3 Hz. The PD group displayed marked coordination problems over and above the known clinical motor symptoms of PD. The performance of both the in-phase and antiphase modes was significantly affected in the PD group compared to the control group; furthermore, the variability of relative phase was significantly increased in this group. These observations were not caused by problems to synchronize the movements with the external pacing signal. In addition to the bimanual coordination problems, involuntary mirror movements (MM) were observed in the single-arm control trials that were significantly larger in the PD group (4.4% of the amplitude of the moving arm) than in the control group (2.3%), suggesting a reduced ability to suppress a basic in-phase coupling of the arms. In the PD group, MM were largest during movements of the least-affected arm. These parkinsonian coordination problems are interpreted in terms of recent evidence on the neural organization of bimanual coordination, suggesting that they are due to cortical rather than callosal dysfunction.