Hand therapists may notice a patient's tremor when treating another diagnostic problem, such as arthritis or a fracture. In these instances, the tremor may become apparent as the patient attempts to don or doff a splint or to practice a home exercise program, or it may be reported in terms of difficulty with dressing or eating. The authors hypothesized that limb cooling would temporarily improve hand function among patients with essential tremor (ET) and that limb warming would temporarily improve hand function among patients with resting tremor secondary to Parkinson disease (PD). Twenty patients with ET and 20 patients with PD completed this single-blind randomized crossover study. Scores following exposure to cold water were compared with scores following exposure to warm water. For patients with ET, subtest scores for the Archimedes spiral, simulated feeding, and checkers were, statistically, significantly lower (i.e., improved) following exposure to cold water than following exposure to warm water; scores for Archimedes spiral card turning, simulated feeding, and checkers were significantly lower following exposure to cold water than at baseline. Scores for Archimedes spiral and card turning were also significantly lower following exposure to warm water than at baseline. For patients with PD, no statistically significant differences were noted between treatments or from baseline except the score for small common objects, which was lower (improved) following exposure to warm water than at baseline. The significant findings from this study support the therapeutic use of cooling to temporarily decrease tremor, thereby improving hand function among patients with ET.