Although problems in bilateral simultaneous movements in Parkinson's disease (PD) are well known, such deficits have not been reported to be any more impaired than simultaneous movements within the same limb. This is surprising, since (a) the parallels between supplementary motor area (SMA) damage and PD are well documented and (b) the SMA seems to play a special role in bilateral motor control. Bilateral versus unilateral movements in PD were examined by using a task that compared alternating movements of fingers of the same hand with alternating movements of fingers of the opposite hands. PD patients showed particular problems in programming and transferring motor activity to fingers on the opposite side of the body, as opposed to switching motor activity between fingers on the same side of the body. These findings outline the relevance of SMA dysfunction to PD.