Nearly half the fundraisers in the 2003 Cancer Research UK Race for Life in Wales did not send in their sponsorship money, the charity has revealed.
Race director Jo Symington said the charity had made the figure public to encourage a better response from runners in this year's event, which involves a series of races across the UK between May and July.
She said: "We don't think that the issue is that women forget to send us their sponsorship money, but that many women do not raise money for the charity at all and take a place at a race that could be filled by a genuine fundraiser."
Collection rates vary across the UK. For instance, in Wales 70 per cent of people taking part in Aberystwyth send in the money compared to only 54 per cent in Cardiff.
Despite the problems, £17.5m was raised across Britain in 2003. Cancer Research UK expects 325,000 women to take part in this year's races.
The British Heart Foundation was forced to take action after its schools skipping challenge, 'Jump Rope for Hearts', only pulled in half of the money pledged.
"We tried to get our regional fundraisers to make contact with schools and to build up a relationship," said head of events Julie Sorrell. "We now send reminders, but it costs money and we have to decide whether it is worth it." She added that remittance rates had improved to 80 per cent.
For its sponsorship of the 'London to Brighton Bike Ride', BHF offers incentives of priority places in next year's event if riders get their sponsorship money in early.
Sense director of fundraising Paul Amadi said: "Obtaining money can often be harder than taking part in fundraising events in the first place."