Beckwith-Wiedemann syndrome (BWS) is a congenital overgrowth syndrome with variable expression. To define the range and frequency of complications in BWS, we have studied a cohort of 76 affected patients (two previously reported). The most frequent complications were macroglossia (97%), abdominal wall defect (80%) and birth weight or postnatal growth > 90th centile (88%). Other common features were ear creases/pits (76%), facial naevus flammeus (62%), nephromegaly (59%) and hypoglycaemia (63%). Rarer complications included hemihypertrophy (24%), moderate/severe developmental delay (4%), congenital heart defects (6.5%), polydactyly (4%), neoplasia (4%) and cleft palate (2.5%). Pre-term labour occurred in 53% and polyhydramnios in 33% of BWS pregnancies. The six deaths all occurred in babies born pre-term, three of whom had major congenital abnormalities. Five patients (6.5%) from four kindreds had an unequivocal family history of BWS, but 15 of 68 apparently sporadic cases had a relative with possible BWS (minor features only). Incomplete penetrance may lead to familial BWS being underdiagnosed.