OBJECTIVES--To measure the incidence of eclampsia, establish how often it is preceded by signs of pre-eclampsia, document the morbidity associated with eclampsia, and determine the maternal case fatality rates. DESIGN--A prospective, descriptive study of every case of eclampsia in the United Kingdom in 1992. Information was collected from reviews of hospital case notes and questionnaires to general practitioners. SETTING--All 279 hospitals in the United Kingdom with a consultant obstetric unit. RESULTS--Obstetricians and midwives notified 582 possible cases, and 383 were confirmed as eclampsia. The national incidence of eclampsia was 4.9/10,000 maternities (95% confidence interval 4.5 to 5.4). Most convulsions occurred despite antenatal care (70%) and within one week of the woman's last visit to a doctor or midwife (85%). Three quarters of first seizures occurred in hospital, of which 38% developed before both proteinuria and hypertension had been documented. Forty four per cent of cases occurred postpartum, more than a third (38%) antepartum, and the remainder (18%) intrapartum. Nearly one in 50 women (1.8%) died, and 35% of all women had at least one major complication. The rate of stillbirths and neonatal deaths was 22.2/1000 and 34.1/1000, respectively. Preterm eclampsia occurred more commonly antepartum and was associated with more maternal complications and fetuses that were small for gestational age, as well as with higher rates of stillbirth and neonatal mortality. Antepartum eclampsia, which was more likely to occur preterm, was associated with a higher rate of maternal complications and a higher neonatal mortality. Both factors (gestational prematurity and antepartum occurrence) contributed independently to the severity of the outcome. CONCLUSION--Eclampsia occurs in nearly one in 2000 maternities in the United Kingdom and is associated with high maternal morbidity and fatality in cases. It may present unheralded by warning signs. Preterm and antenatal eclampsia seem to be particularly severe.