Documents containing about 200 people's financial details were found strewn across a road after they apparently fell off the back of a lorry in Wantage.
Thousands of sheets of bank, mortgage and pension details, as well as names, addresses, phone numbers and dates of birth, were found in Mill Street on Monday.
Builder Ryan Tate was shocked when he discovered the information belonged to people from across the country.
The 34-year-old picked up the documents, strewn over 70 yards from four boxes, and handed them in to police.
He said: "They must have fallen off the back of a van. It looked like it had gone round the corner too fast. There was so much information.
"There were four boxes of financial information from lots of places, including the Co-op and Norwich Union, and I saw people's details who were from Didcot and Birmingham, so it must have been from all over the country."
Mr Tate, from Aldershot, in Hampshire, was worried about what could have happened to the information in the wrong hands.
He said: "I saw the details of one account which had £200,000 in it.
"If you were the right sort of person, you could have had a field day with it.
"It was all sorts of information and if you were unscrupulous you could probably make a bit of money and could even become someone else.
"It looked like it was going for destruction, but how many other files could have been lost further up the road?
"I wouldn't have been happy if my details were left there."
He added: "It shouldn't have come off the back of a lorry. It should have been tied down."
Police spokesman Toby Shergold said they managed to trace the owners of the information and that it had been on its way to storage via a courier company.
He said the information had been mislaid in transit but refused to name the company. He added: "We would like to thank Mr Tate for his responsible actions."
A spokesman from the Information Commissioner's Office, a Government agency with responsibility for protecting personal information, said it could not comment on the case but said breaches of people's privacy were taken very seriously.
She said: "Where personal information is not processed securely, there's a risk it can fall into the hands of criminals.
"It's essential that organisations transferring personal information do so securely."
She added: "If we receive further details, such as the name of the organisation involved, we will look into it."
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