This experiment tested the hypothesis that Parkinson's disease (PD) patients are more vulnerable to a moderate level of secondary task load than elderly or young controls due to heightened variability in the motor system. PD patients, elderly, and young adults performed a handwriting task with different secondary tasks. The secondary task imposed motor load (i.e., speech) and/or a mental load (i.e., ignoring, repeating, or subtracting). The findings showed that, in contrast to young and elderly controls, PD patients tended to increase MT, accumulated pause time, and normalized jerk when the secondary task consisted primarily of motor load. Furthermore, it was shown that PD patients did not reduce writing sizes as result of a high level of mental load which finding suggests that writing in an automated fashion does not result in micrographia. The results are discussed in relation to strategies imposed to contend with reduced signal-to-noise levels in the motor system.