Photosensitivity has proved to be a useful model to study the acute effects of experimental antiepileptic drugs (AEDs). The photosensitivity range is usually diminished or even abolished after administration of a known or experimental AED. An increase in photosensitivity, an unexpected reaction, was found in four photosensitive epileptic patients after oral ingestion of 500, 100, or 50 mg of Org 6370. Moreover, the three patients receiving doses of 100 and 500 mg reported nausea, dizziness, restlessness, and an increase in spontaneous epileptic seizures (myoclonus and in one patient a generalized tonic-clonic convulsion). The side effects coincided with peak Org 6370 serum levels. Our findings indicate that in the photosensitivity model experimental drugs with proven anticonvulsant properties in animals may increase rather than decrease the degree of patient photosensitivity. Photosensitive patients may represent a special subgroup of epileptic patients and therefore need to be classified as such.