We evaluated a kindred with X‐linked mental retardation and epilepsy. Seven affected males with mild to moderate mental retardation developed seizures (primarily generalized, tonic–clonic, and atonic) that began on average at 6.8 months of age (range, 4 to 14 months). These patients did not have a history of infantile spasms. There were no dysmorphic features. Other than mental retardation, the neurological examination was unremarkable, with exception of 2 affected subjects who had mild generalized rigidity and ataxia. We identified tight linkage to a group of markers on Xp21.1–p11.4. A maximum two‐point LOD score of +3.83 at θ = 0 was obtained for markers DXS8090, DXS1069, DXS8102, and DXS8085. This locus spans 7.7cM between DXS1049 and DXS8054 and does not overlap the locus for X‐linked West syndrome. The tetraspanin gene, implicated in nonspecific mental retardation, is mapped to this region. We sequenced the tetraspanin coding sequence in subjects with X‐linked mental retardation and epilepsy and did not identify disease‐specific mutations. The syndrome we describe, designated X‐linked mental retardation and epilepsy, is clinically and genetically distinct from X‐linked West syndrome and other X‐linked mental retardation–epilepsy syndromes.