The clinical, electromyographic and electroencephalographic features in a case of presenile dementia associated with myoclonus and running a slowly progressive course are described. Photic-induced seizures occurred in spite of nonspecific background EEG changes, and in the absence of spontaneous seizures. A brain biopsy was consistent with the diagnosis of Alzheimer''s disease. The case is discussed with respect to the diagnostic problems of the association between presenile dementia and myoclonus. The need for routine photic stimulation in these patients is stressed.