PARKINSON'S DISEASE /improving motor-skill control. Parkinson's support groups recommend T'ai Chi, and many students claim significant reduction in tremors. 
Parkinson's disease. A 33-person pilot study from Washington University School of Medicine in St. Louis, published in Gait and Posture (October 2008), found that people with mild to moderately severe Parkinson's disease showed improved balance, walking ability, and overall well-being after 20 tai chi sessions.
Harvard Medical School's Health Publications, May, 2009
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QIGONG HELPS REDUCE THE MOTOR & NON-MOTER SYMPTOMS OF PARKINSON'S DISEASE
Schmitz-Hubsch et al. of Germany published their study in a recent issue of Movement Disorders: Qigong exercise helps reducing the motor and non-motor symptoms of Parkinson's disease: A randomized controlled pilot study. 
ABSTRACT: Irrespective of limited evidence, not only traditional physiotherapy, but also a wide array of complementary methods are applied by patients with Parkinson's disease (PD). We evaluated the immediate and sustained effects of Qigong on motor and nonmotor symptoms of PD, using an add-on design. Fifty-six patients with different levels of disease severity (mean age/standard deviation [SD], 63.8/7.5 years; disease duration 5.8/4.2 years; 43 men [76%]) were recruited from the outpatient movement disorder clinic of the Department of Neurology, University of Bonn. We compared the progression of motor symptoms assessed by Unified Parkinson's Disease Rating Scale motor part (UPDRS-III) in the Qigong treatment group (n = 32) and a control group receiving no additional intervention (n = 24). Qigong exercises were applied as 90-minute weekly group instructions for 2 months, followed by a 2 months pause and a second 2-month treatment period.
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Benefits were perceived by participants in physical, psychological, and social domains. Thirteen of the survey respondents, including 6 of the 8 respondents with Parkinsons reported perceiving a physical benefit attributed to tai chi practice. Improved balance was reported most frequently. Instructor observations and participant testimony suggest movement capability for individuals with Parkinsons may also be improved by performing tai chi. 
-- Journal of Neurology and Physical Therapies, 30, 22-7