Multiple gestation pregnancy rates are high in assisted reproductive treatment cycles because of the perceived need to stimulate excess follicles and transfer excess embryos in order to achieve reasonable pregnancy rates. Perinatal mortality rates are, however, 4-fold higher for twins and 6-fold higher for triplets than for singletons. Since the goal of infertility therapy is a healthy child, and multiple gestation puts that goal at risk, multiple pregnancy must be regarded as a serious complication of assisted reproductive treatment cycles. The 1999 ESHRE Capri Workshop addressed the psychological, medical, social and financial implications of multiple pregnancy and discussed how it might be prevented. Multiple gestations are high risk pregnancies which may be complicated by prematurity, low birthweight, pre-eclampsia, anaemia, postpartum haemorrhage, intrauterine growth restriction, neonatal morbidity and high neonatal and infant mortality. Multiple gestation children may suffer long-term consequences of perinatal complications, including cerebral palsy and learning disabilities. Even when the babies are healthy they must share their parents' attention and may experience slow language development and behavioural problems. Current data indicate that the average hospital cost per multiple gestation delivery is greater than the average cost of in-vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) cycles. Prevention is the most important means of decreasing multiple gestation rates. Multiple gestation rates in ovulation induction and superovulation cycles can be reduced by using lower dosage gonadotrophin regimens. If there are more than three mature follicles, the cycle should be converted to an IVF cycle, or it should be cancelled and intercourse should be avoided. In IVF cycles two embryos can be transferred without reducing birth rates in most circumstances. Embryo reduction involves extremely difficult decisions for infertile couples and should be used only as a last resort. Assisted reproductive treatment centres and registries should express cycle results as the proportion of singleton live births; twin and triplet rates should be reported separately as complications of the procedures. Reducing the multiple gestation pregnancy rate should be a high priority for assisted reproductive treatment programmes, despite the pressure from some patients to transfer more embryos in order to improve success. If nothing is done, public concern may lead to legislation in many countries, a step that would be unnecessary if assisted reproductive treatment programmes and registries took suitable steps to reduce multiple pregnancy rates.