Trisomy 18 (Edwards' syndrome) presents with characteristic external features as well as life-threatening abnormalities; many of these abnormalities require surgical correction during the neonatal period. Children with trisomy 18 have a very short life expectancy, and all long-term survivors have severe mental retardation. Difficult medical and ethical issues arise over whether or not to institute treatment when a newborn infant with suspected trisomy 18 has a life-threatening anomaly. We studied the policy of treatment in seven patients with clinical Edwards' syndrome. For three, the period of uncertainty was shortened because trisomy 18 was rapidly diagnosed by karyotyping of a bone-marrow aspirate. Four of the patients underwent surgery before the diagnosis of trisomy 18 was confirmed by routine karyotyping in lymphocytes; karyotyping in bone marrow might have allowed invasive treatment to be avoided in three of these. Rapid confirmation of clinically suspected Edwards' syndrome is very important because surgery may then be withheld. A newborn infant with trisomy 18 should be considered as a patient with a hopeless outlook who ought not to be subjected to invasive procedures. The decision to withdraw or withhold treatment should be discussed frankly with the parents. The period of uncertainty can be reduced to a minimum by the use of karyotyping in bone marrow.