Objective: To determine whether the volume of the putamen is abnormal in patients with idiopathic focal dystonia. Background: The cause of adult- onset focal dystonia is unknown, but substantial evidence suggests that the putamen may be abnormal in this condition. Cell loss and gliosis have been suggested. We hypothesized that this might be reflected as abnormal putamen volume on MRI. Design and methods: A high-resolution MRI was acquired in 13 adults with cranial or hand dystonia and 13 normal individuals matched for age and sex. Putamen volume was measured using a stereologic method (Study 1). In a replication study, another rater measured putamen volume using manual tracing and direct voxel count (Study 2). Neither rater was aware of the diagnosis, and the order of measurement was random in each study. Results: In Study 1, putamen measurements were reasonably accurate (coefficient of error, ~6%). The putamen was 13% larger in patients, both in absolute terms (p = 0.03) and after covarying total brain volume (p = 0.02). In Study 2, putamen volumes correlated with those measured in Study 1 (intraclass correlation coefficient, 0.68 to 0.83). The putamen was 8% larger in patients (p = 0.06) and was larger in the patient than in the matched control subject in 10 of 13 pairs (p = 0.046). Conclusion: We find no evidence of putaminal atrophy or degeneration in adult-onset idiopathic focal dystonia. In fact, in this group, the putamen is about 10% larger in patients than in matched control subjects. This finding may reflect a response to the dystonia or may relate to its cause.