Male cyclists are twice as likely to suffer from impotence as those who stay away from bikes, according to a new study.
Cyclists are warned the sport could cause health problems
The report also found that certain saddles can cause health problems for women cyclists.
Its publication coincides with the start of Bike Week, during which thousands of events will be held to encourage Britons to do more cycling.
Organisers are targeting the country's 10 million 'returning' cyclists - people who own a bike but rarely use them.
Researchers from University Hospital in Brussels tested a thousand cyclists to find out how the activity affects the body.
They found that more than 60% of men and women reported genital discomfort.
Bike Week is aimed at Britain's 'returning' cyclists
Their 'Bicycle Saddle Report 2003' suggests that male cyclists are twice as likely to suffer from virility problems.
For women riders, genital swelling was much more common than previously realised.
The researchers said a new design of narrow saddle with a hole in the middle can cause temporary numbness for women.
They recommend that cyclists sit upright whenever possible and stand on the pedals every 10 minutes.
Organisers of Bike Week will hope the findings do not put people off cycling.
During the week six new sections of the national cycle network will be opened by transport charity Sustrans, in areas including Basingstoke, the Isle of Wight, Cumbria, Renfrewshire and Rugby.
On Sunday, the 28th annual London to Brighton bike ride is taking place in aid of the British Heart Foundation.
Almost 30,000 riders were expected to take part in the event.