Summary Background No studies have had sufficient size to estimate mortality in children with febrile seizures. We studied mortality after febrile seizures in a large population-based cohort of children in Denmark with up to 28 years of follow-up. Methods We identified 1 675 643 children born in Denmark between Jan 1, 1977, and Dec 31, 2004, by linking information from nationwide registers for civil service, health, and cause of death. Children were followed up from 3 months of age, until death, emigration, or Aug 31, 2005. We estimated overall and cause-specific mortality after first febrile seizures with survival analyses. Furthermore, we undertook a case-control study nested within the cohort and retrieved information from medical records about febrile seizure and neurological abnormalities for children who died (N=8172) and individually-matched controls (N=40 860). Findings We identified 8172 children who died, including 232 deaths in 55 215 children with a history of febrile seizures. The mortality rate ratio was 80% higher during the first year (adjusted mortality rate ratio 1·80 95% CI 1·31–2·40) and 90% higher during the second year (1·89 1·27–2·70) after the first febrile seizure; thereafter it was close to that noted for the general population. 132 of 100 000 children (95% CI 102–163) died within 2 years of a febrile seizure compared with 67 (57–76) deaths per 100 000 children without a history of this disorder. In the nested case-control study, children with simple (≤15 min and no recurrence within 24 h) febrile seizure had a mortality rate similar to that of the background population (adjusted mortality rate ratio 1·09 95% CI 0·72–1·64), whereas mortality was increased for those with complex (>15 min or recurrence within 24 h) febrile seizures (1·99 1·24–3·21). This finding was partly explained by pre-existing neurological abnormalities and subsequent epilepsy. Interpretation Long-term mortality is not increased in children with febrile seizures, but there seems to be a small excess mortality during the 2 years after complex febrile seizures. Parents should be reassured that death after febrile seizures is very rare, even in high-risk children. Funding The Danish Research Agency, Lennart Gram Memorial Trust (Denmark), P A Messerschmidt and Wife's Foundation (Denmark), Managing Director Kurt Bønnelycke and Mrs Grethe Bønnelyckes Foundation (Denmark).