Spam e-mails tipping shares in small companies have increased by nearly a third between December and March, according to industry experts.
Many of the shares recommended are either worthless or selling at massively inflated prices, spam detection firm Clearswift warned.
People who buy on the back of the bogus recommendations could make heavy losses as a result.
Most of the shares are in technology and media firms, Clearswift added.
"The spammers are looking at a new angle to cash in and it is financial services," said Alyn Hockey, director of research at Clearswift.
Mr Hockey added that the typical bogus investment e-mail carried the tell-tale signs of spam such as nonsense words to fool corporate spam filters.
However, the firm said the level of share-related spam e-mails was well below the torrent of unsolicited offers for sexual aids and pornography.