Progressive myoclonus epilepsy (PME) is a syndrome complex encompassing different diagnostic entities. Among the 30 cases of PME studied during 1982 and 1992 at the National Institute of Mental Health and Neurosciences, Bangalore, South India, the specific diagnoses included Lafora disease (LD), neuronal ceroid lipofuscinosis (NCL). Unverricht-Lundborg disease (ULD), and myoclonus epilepsy and ragged-red fibres (MERRF). We discuss the familial nature of PME and the clinical and electrophysiological abnormalities in asymptomatic siblings. Eight cases of LD were in three different families with 3 affected siblings in two families (L1, L2) and 2 siblings in the third family (L3). Occipital seizures and behavioral changes occurred in all 3 members of L1 but were absent in the other two families. Age of onset was similar in two families (L1, 11 years; L2, 14.5 years), but not in the third. Presymptomatic EEG abnormalities were observed as long as 6 years before onset in L2. ULD occurred in 2 sisters in one family. Both had identical clinical features and normal somatosensory evoked potentials (SSEPs). The asymptomatic sister of the patient MERRF had abnormal EEG and giant SSEPs for the past 2 years. Thus, although all variations are evident in the overall clinical pattern in each of the PME, affected member of individuals families tend to be similar. Once an index case is identified, electrophysiologic tests (EEG and SSEP) may be useful in identifying other affected siblings in the presymptomatic stage.