Post-mortem investigation of sudden death in young people frequently reveals no overt cause for the death. Full investigation is hampered if tissue or blood is not retained for DNA analysis. We report a post mortem molecular diagnosis of long QT syndrome in a 12-year-old boy diagnosed with epilepsy who died suddenly playing sport. The DNA was extracted from an archived blood spot on his newborn screening ('Guthrie') card, which had been taken from him at 6 days of age. A missense mutation was detected in exon 5 of the KCNQ1 gene; R243C (835C > T), associated with long QT type 1. The same mutation was found in the mother (who now takes effective preventative therapy), but not in the sib who has now been reassured that she is not at risk of sudden death.