We identified a family with three males in two generations with moderate mental retardation. The two oldest were first cousins whose mothers were sisters. The third affected was a grandson through a daughter of one of the sisters, strongly suggesting Xlinked inheritance. The affected males had prominent glabella, synophrys, prognathism, generalized hirsutism, and bilateral single palmar creases. All developed seizures in childhood. The two oldest have had a slow deterioration in neurological status with poor gait and balance and progressive weakness. No deterioration in their mental status has been observed. The oldest had cerebellar atrophy confirmed on computed tomography and magnetic resonance imaging scans of the brain and prolonged nerve conduction velocity. Two of the males had hypogammaglobulinemia (IgA deficient). Two-point linkage analysis using 27 microsatellite markers on the X chromosome resulted in a maximum LOD score of 2.23 at = 0 for locus DSX101. Recombination was observed at locus DSX1170 in Xq21.33 and locus DXS8067 in Xq23. We conclude that this family represents an X-linked disorder asso-ciated with a recognizable phenotype, progressive neurological deterioration, and variable hypogammaglobulinemia. The gene appears to lie between Xq21.33 and Xq23. Am.