About 150 persons who have PD and low vitamin D levels will participate in this study. Subjects will be randomly (like flipping a coin) assigned to either high dose vitamin D supplement (54,200 IU weekly) or the Recommended Daily Allowance (RDA) for older persons (4200 IU weekly of vitamin D). Subjects will be examined in the clinic before, then 3- and 6- months after taking vitamin D supplement. Tests of walking speed, Parkinson's rating scales, memory tests and questionnaires of mood, anxiety and fatigue will be administered.