OBJECTIVE: To apply and validate the use of electromyogram (EMG) recorded during functional magnetic resonance imaging (fMRI) in patients with movement disorders, to directly relate involuntary movements to brain activity. METHODS: Eight "familial cortical myoclonic tremor with epilepsy" (FCMTE) patients, with tremor-like cortical myoclonus and cerebellar Purkinje cell degeneration, and nine healthy controls performed hand posture and movement in an on/off fashion (block design). Superfluous movements were quantified as deviations in EMG power, positive and negative, with respect to the average EMG per session. This measure, "residual EMG" (r-EMG), was derived by Gram-Schmidt orthogonalization. Activation maps resulting from conventional block regressors and novel r-EMG regressors were compared. RESULTS: In healthy participants, the block posture regressor identified mainly cerebellar activity and some activity in other areas belonging to motor circuitry. In FCMTE patients, no cerebellar activity was seen with the block posture regressor, compatible with cerebellar Purkinje cell changes in FCMTE. EMG power showed little variation during posture in healthy controls. Therefore, the r-EMG regressor was almost constant and revealed no brain activity as expected. In contrast, in FCMTE patients the r-EMG posture regressor was highly variable due to continuous myoclonic jerks. It identified sensorimotor cortical areas, compatible with cortical hyperexcitability in FCMTE patients. CONCLUSION: Conventional block regressors can be used to identify neuronal circuitry associated with a specific motor task, whereas r-EMG regressors can help identify brain activation directly related to involuntary movements. Simultaneous EMG-fMRI is complementary to conventional fMRI and will facilitate studies of hyperkinetic movement disorders.