Although the literature contains several references to clinically apparent cognitive deficits in patients with myotonic dystrophy (MYD), efforts to support these observations with formal testing have been lacking. The current study compared 17 MYD patients with 25 normal controls on an expanded Halstead-Reitan Battery. The MYD group scored worse than the controls on nearly every neuropsychological measure. Significant neuropsychological impairment was present even when tests of motor skills were excluded. There was no relationship between general neuropsychological impairment and degree of weakness, myotonia, or muscle atrophy in the MYD patients. These findings suggest that cognitive impairment can be an important and relatively independent component of the disability in MYD, which should be considered in the clinical evaluation and counselling of persons with this disease.