Iris pigmentation was evaluated in 153 Caucasian patients with torticollis and other focal, segmental, or generalized dystonias of unknown cause. Since these disorders are rare in non-Caucasians, it was hypothesized that a relationship might exist between decreased melanin metabolism, reflected by iris pigmentation, and a genetic predisposition to these disorders of voluntary movement. Patients were separated into two groups on the basis of iris pigmentation. Compared to control groups, there was a statistically significant reversal in the ratio of patients with light eyes versus dark eyes. Further comparisons were made in two groups of patients with drug-induced nonparkinsonian dyskinesias; no significant correlation was found between their dyskinetic syndrome and iris pigmentation. Data concerning race, ethnic origin, and other disorders of the basal ganglia support the hypothesis that there may be genetic predilection in individuals whose metabolic pathways produce less systemic melanin.