Men who use laptop computers could be unwittingly damaging their fertility, experts believe.
Using it on your lap could be risky
Balancing it on the lap increases the temperature of the scrotum which is known to have a negative effect on sperm production, researchers found.
Coupled with the rising popularity of these computers - about 150m people use them worldwide - much more research is needed say the US authors.
The State University of New York findings appear in Human Reproduction.
Lead researcher Dr Yefim Sheynkin, from the State University of New York at Stony Brook, said: "Laptops can reach internal operating temperatures of over 70°C.
"They are frequently positioned close to the scrotum, and as well as being capable of producing direct local heat, they require the user to sit with his thighs close together to balance the machine, which traps the scrotum between the thighs."
The researchers asked 29 healthy male volunteers aged between 21 and 35 to take part in an experiment.
They then recorded the temperature changes to the scrotum caused by laptop use and different seating positions over one hour time periods.
Just sitting with the thighs together, a posture needed to balance a laptop, caused scrotal temperatures to rise by 2.1C.
When the men used a laptop in this position the average temperatures increased by 2.6C on the left of the scrotum and 2.8C on the right.
Dr Sheynkin said: "The body needs to maintain a proper testicular temperature for normal sperm production and development (spermatogenesis).
"We don't know the exact frequency and time of heat exposure capable of producing reversible or irreversible changes in spermatogenesis.
"But previous studies suggest that 1°C above the baseline is the possible minimal thermal gradient."
He said any changes might be reversible, but that repetitive use of a laptop in this way might cause permanent damage.
"Until further studies provide more information on this type of thermal exposure, teenage boys and young men may consider limiting their use of laptop computers on their laps," he said.
Speaking on behalf of the British Fertility Society, Dr Allan Pacey, senior lecturer in Andrology at the University of Sheffield, said: "We already know that increasing the temperature of the testicles can affect sperm production.
"It is worrying that having a laptop on your knees for only an hour can increase the temperature of the scrotum so significantly.
"Men who use laptops regularly should be very careful.
"Further work is needed to see if regular laptop use is a risk factor in male fertility."