Significant variation in the neuropathological abnormalities associated with X-linked aqueductal stenosis has been found in males from two unrelated families. Family A is the first reported family with this disorder. A recently born male infant had stenosis and forking of the aqueduct, whereas his uncle had only stenosis. In family B one male had extensive midline fusion of the thalami in addition to aqueductal stenosis and forking. These findings of phenotypic variability indicate broader criteria for establishing the diagnosis of X-linked recessive aqueductal stenosis.