Hydrolethalus syndrome is a severe complex malformation syndrome, which leads to death of the fetus in most cases in utero, or immediately after the birth. The causative gene, still waiting to be cloned, is located at chromosome 11q23-q25. The characteristic malformations are external asymmetric hydrocephalus, a keyhole-shaped foramen magnum and polydactyly in all extremities. Other common findings are poorly developed mandible, small, often bifid nose and hypoplastic eyes. Atrioventricular communis defect of the heart is frequently found as well as abnormal lobation of the lungs. Careful clinical examination provides the diagnosis of the hydrolethalus syndrome distinct from Meckels syndrome. Prenatal diagnosis by ultrasound can be established already at 12th week of gestation.