X-linked hydrocephaly is a rare sex-linked genetic recessive condition occurring in 1/30,000 deliveries. Adduction of thumbs and mental retardation are additional associated clinical findings. We describe two cases of X-linked hydrocephaly with associated adducted thumbs that were diagnosed prenatally with the combined use of three-dimensional (3D) ultrasound and fetal blood sampling for cytogenetic and molecular analyses. This report suggests that 3D ultrasound can facilitate the identification of adducted thumbs in fetuses affected by X-linked hydrocephaly and supports evaluation of the fetal hands as an integral part of the ultrasound anatomical assessment in male fetuses with hydrocephaly secondary to aqueductal stenosis.