More than 230,000 elderly people in Japan who are listed as being aged 100 or over are unaccounted for, officials said following a nationwide inquiry.
An audit of family registries was launched last month after the remains of the man thought to be Tokyo's oldest were found at his family home.
Relatives are accused of fraudulently receiving his pension for decades.
Officials have found that hundreds of the missing would be at least 150 years old if still alive.
The Justice Ministry said some of those unaccounted for may have died as long ago as World War II, possibly during the post-war turmoil.
Others may have emigrated without reporting their status to local authorities, or relatives simply did not report the deaths.
The inquiry followed the discovery of the mummified remains of Sogen Kato, who was thought to be the oldest man in Tokyo.
However, when officials went to congratulate him on his 111th birthday, they found his 30-year-old remains, raising concerns that the welfare system is being exploited by dishonest relatives.
Reports said he had received about 9.5m yen ($109,000; £70,000) in pension payments since his wife's death six years ago, and some of the money had been withdrawn.
Japan has one of the world's fastest ageing societies, with one in five over the age of 65.
Last year's Health Ministry report said Japan had 40,399 people aged 100 or older with known addresses.