Harmful electromagnetic radiation from the wireless network used to send signals to laptop computers and mobile phones risks leaving men infertile, scientists believe.
Researchers took sperm from 29 men aged 26 to 45 and placed them either under a wi-fi connected laptop or away from the computer.
The laptop then uploaded and downloaded information from the internet for four hours.
At the end of the experiment, 25 per cent of the sperm under the laptop had stopped moving and nine per cent showed DNA damage.
By comparison, just 14 per cent of samples kept away from the wi-fi stopped moving. And just three per cent suffered DNA damage.
The study, by U.S. and Argentine scientists, was reported in the journal Fertility and Sterility.
"Our data suggests that the use of a laptop computer wirelessly connected to the internet and positioned near the male reproductive organs may decrease human sperm quality," said lead researcher Conrado Avendano, of Nascentis Medicina Reproductiva in Cordoba.
"At present we do not know whether this effect is induced by all laptop computers connected by WiFi to the internet or what use conditions heighten this effect."
A separate test with a laptop that was on, but not wirelessly connected, found negligible EM radiation from the machine alone.
The findings fuel concerns raised by a few other research teams.
Some have found that radiation from mobile phones creates feeble sperm in the lab, for example.
But Dr Robert Oates, the president of the Society for Male Reproduction and Urology, who has fathered two kids despite having both a laptop and an iPad, sounded a note of reassurance.
"This is not real-life biology, this is a completely artificial setting," he told Reuters Health.