The rate of sudden unexplained deaths in England is around eight times higher than previously thought, warn experts.
Around 500 people may die every year from sudden arrhythmic death syndrome, a study published in Heart shows.
SADS is linked to a genetic heart defect and family members should be screened to prevent more deaths, the researchers said.
The study also found that only one-third of cases had been correctly identified by post-mortem.
The researchers identified 56 cases of SADS from 115 coroners' reports of unascertained causes of death.
None of those who died had a history of heart disease, and they had all last been seen alive within 12 hours of death.
The average age of death was 32 and 63% were men.
Four had had some heart symptoms in the 48 hours before death, and two -thirds had experienced cardiac symptoms at some point in the past.
From their sample, the researchers calculated that the total annual numbers of SADS cases per 100,000 of the population was 0.16.
This figure was higher than the number of SADS deaths listed in national statistics, at 0.10 per 100,000 of the population.
But, when the researchers added up all the unknown causes of death in national records that might have actually been SADS, they uncovered a potentially much bigger discrepancy.
They found the rate could be as high as 1.34 per 100,000 - up to eight times higher than they had estimated and equating to 500 deaths per year.
Underreporting of SADS could be due to deaths being misclassified, inconsistency in referral by coroners or families not agreeing to further expert cardiac examination, they explained.
Some of the deaths in the study were attributed to heart attack or other causes, such as epilepsy and drowning.
Almost one in five SADS cases had a family history of sudden unexplained deaths before the age of 45.
Previous research by the team showed a 22% incidence of underlying inheritable cardiac disease.
The team concluded that SADS should be a certifiable cause of death and that affected families should be screened by a specialist.
"Deaths from SADS occur predominantly in young males," the researchers concluded. "When compared with official mortality, the incidence of SADS may be up to eight times higher than estimated.
"Families with SADS carry genetic cardiac disease, placing them at risk of further sudden deaths."
Ellen Mason, a British Heart Foundation heart nurse, said: "Clearer ways to identify possible victims of SADS are vital.
"If a person dies from SADS, specialist centres can offer genetic screening to their bereaved families. Monitoring people who could be at risk of SADS and giving them specialist treatment may prevent further tragic deaths."
"By underestimating the number of deaths caused by SADS every year, families who might be at risk may slip through the net and this may result in further tragedies."
Anne Jolly, from SADS UK, said the charity heard from many families left devastated and bewildered after the premature sudden and unexpected death of an apparently healthy child or spouse.
She added: "When there is no cause of death given this adds to their confusion and pain.
"Some of these conditions are genetic and it is important that other family members seek specialist advice as they too may be at risk of death from the same genetic condition."