Organisers of a series of charity fun runs have urged participants to collect sponsorship, after 40% of runners failed to return any money last year.
Race for Life runners have raised £70m for Cancer Research
Cancer Research is currently recruiting women to take part in Race for Life events around the UK.
In 2004, thousands of runners had to be turned away because capacity was full, but only 60% of runners actually sent in money afterwards.
Eight Race for Life events are being held in Wales, with two in Cardiff.
Cancer Research spokeswoman Kate Stocken said the percentage of runners sending in money had risen from 2003 - when 46% of people did not return any cash - but there was still room for improvement.
In the 11 years since Race for Life began in 1994, 1.4 million women have taken part, raising £70m.
This year, organisers hope to collect £23m to research the causes, treatment and prevention of cancer.
In a letter, the charity tells runners that their entry fee "only covers the cost of putting on the event, from first aid cover to course set-up, marshalling and toilets".
Lucinda Frostick, communications manager from the Institute of Fundraising, said: "If you are taking part in any fundraising event you want to make the most out of it, to put the effort in because it's worth so much more to the charity at the end of the day.
"For people who are taking part in the race, they need to make the effort to collect [their sponsorship money]."