A diagnosis of infantile sialic acid storage disease was made in an infant who died aged 17 months. In the mother's next pregnancy no morphological or biochemical abnormality was found in chorionic villi, amniotic fluid, cultured amniotic fluid cells or fetal blood and a normal boy was born. In the subsequent pregnancy an ultrasound scan revealed a twin pregnancy. Chorionic villus samples were obtained from both twins and microscopic and biochemical analysis indicated one twin to be affected with sialic acid storage disease. Selective fetocide was performed. The unaffected twin proceeded to term.