More than a third of all spam e-mails originate in the US, according to a survey by security firm Sophos.
Spam: Made in the USA?
Computers in the US were responsible for more than 35% of global spam, while South Korea was responsible for almost a quarter of all spam e-mails.
According to Sophos more than 50% of all spam is sent from "zombie" PCs, computers that have been take over by hackers or virus writers.
The survey comes as spam is reported to be on the wane.
The survey was compiled using the firm's "global network of spam traps".
"Although the USA is responsible for producing more than a third of all of the world's spam, its percentage contribution has decreased," said Graham Cluley, senior technology consultant for Sophos.
Deborah Platt Majoras, the US's top consumer protection official, said on Thursday that data from internet service providers
(ISPs) suggested the deluge of unwanted emails selling sex,
drugs and get-rich quick schemes may be waning.
On a visit to Brussels, the head of the US Federal Trade Commission (FTC), said: "Spam has been a huge problem around the world, not just in these two continents.
"Recently the ISPs seem to think that less spam is actually
reaching consumers' mailboxes. That's the first good news I've
heard on spam, if it's true, since I started."
"The percentage of spam sent from American computers has fallen 12% since the start of the year compared to other countries," said Mr Cluley.
He said the drop was a result of ISPs enforcing anti-spam policies and a growing awareness of the need to protect all computers from being hacked by spammers.
"Businesses and computer users must protect themselves from becoming a zombie by having up-to-date anti-virus software, firewalls and Microsoft security patches," he added.
PCs in China sent almost 10% of spam, said the survey, while countries including France, Spain and the UK were each responsible for between one and three percent of spam e-mails.