Kidney patients needing a transplant could be facing a postcode lottery in accessing organs, research suggests.
A study found large variations across the UK that could not be explained by differences in the health of patients.
Some kidney patients fall off the waiting list if they become too ill for a transplant but, if their health improves, they can go back on.
The average waiting time for an adult kidney transplant is 841 days, but many people die before they receive an organ.
Children, who are prioritised, wait an average of 164 days.
There are currently 6,865 adults and 111 children on the UK waiting list for a kidney transplant.
The study, from the UK Renal Registry and NHS Blood and Transplant (NHSBT) found variations in accessing the list, the time patients had to wait to get on the list, and the time they had to wait for a transplant.
The research, published online in the British Medical Journal (BMJ), studied 16,202 dialysis patients from 65 renal centres across the UK.
Data on the patients, who were all under 65, was submitted to the renal registry between January 2003 and December 2005, with follow-up to December 2008.
The national average percentage of patients registered for a transplant within two years of starting dialysis was 50%. But the study found this varied from about 25% in some centres to 65% in others.