Regional cerebral blood flow (rCBF) during photic myoclonus was studied by means of positron emission tomography in a 51-year-old male patient suffering from MERRF (myoclonic epilepsy with ragged-red fibres). Frequencies of 3 Hz and 5 Hz flash stimulation were used. Both frequencies elicited paroxysmal EEG-abnormalities but only the higher frequency induced generalized myoclonic jerks. We found a left dominated significant increase of rCBF in the thalamus during myoclonus. The thalamic asymmetry was significant when tested with an ROI approach. The myoclonic activity was accompanied by significantly increased activity in the supplementary motor cortex (SMA). Decreases in rCBF were seen in the limbic, temporal and occipital areas during photic stimulation at both flash frequencies, more expressed during myoclonus. The findings observed in this patient indicate a thalamic focus for photic myoclonus in MERRF. We suggest that photic stimulation induced an abnormal discharge in the thalamus, which was fed forwards via thalamo-cortical connections to the precentral motor cortex, to produce the muscle jerks.