Hospitals in England are each charging their patients up to £1.5m a year for car parking, the BBC has learned.
Patients can be charged up to £30 a day for parking
Twelve hospital trusts each raised over £1m in charges, figures obtained under the Freedom of Information Act show.
The Department of Health said it was up to NHS trusts to decide what to charge, and said charges deterred non-service users from using the car parks.
But Macmillan Cancer Relief said it was "morally wrong" to raise revenue by forcing patients to pay for parking.
The hospitals earning the most from car parks tended to be the larger trusts treating more patients.
The Department of Health figures suggest that one trust, University Hospital Birmingham, raised £1.5 million from car parking charges in 2004-5.
But the trust says this is the revenue from the car parks from three health trusts on four hospital sites; the two sites of the University Hospital Birmingham, the Birmingham and Solihull Mental Health Hospital and the Women's Hospital.
A spokeswoman said: "We actually have one of the lowest charges for car parking in the country, this high revenue figure is due to the fact that a lot of people are using these spaces."
The two University Hospital Birmingham sites serve more than 553,000 people a year.
Of the £1.5m revenue, £1m was pumped into the costs of running the car parks and a shuttle bus from the local train station, while the rest was invested back into patient care.
The Cambridge University NHS Foundation Trust also raises more than £1m from its car parks.
The trust's car parks serve two hospitals, the large 1,100-bed Addenbrooke's and the Rosie Hospital for women and charge a flat rate of £2.50.
A spokeswoman said the income paid for improvements such as external lighting, walkways, cycle racks and ways, security and improved car park and travel facilities.
Basildon and Thurrock University Hospitals NHS Foundation Trust raised just over £1m from its car parks over two hospital sites.
A spokeswoman said: "In direct response to local demand, we have considerably expanded the number of spaces available and also ensured a good standard of lighting, security, etc... investing some £1.2m in car park expansion and improvements in the last six years."
A spokeswoman for the Oxford Radcliffe Hospitals NHS Trust, which raised more than £1.2m from its car parks, pointed out it was one of the largest trusts in the country, with approximately 1,200 patient and visitor parking spaces, split across four separate sites.
"The hospital car parks are not run as a commercial operation and all money raised goes directly back into services for patients and visitors.
"The average cost of a three-hour stay at the John Radcliffe Hospital, the biggest of the four sites, is only £1.50."
A Department of Health spokeswoman said parking charges helped hospitals because they discouraged people who were not using the hospital from taking up the car park spaces.
She added that most hospitals had exemptions from charges for patients, "and hospital staff are generally well trained in advising patients about these exemptions".
"Ultimately, it is a matter for individual NHS trusts to decide whether or not to charge for car parking, and the level of charges in the light of local circumstances."
Macmillan Cancer Relief has been campaigning to end hospital parking charges, especially for cancer patients who make frequent hospital trips for treatment.
The charity's chief executive Peter Cardy said: "Raising revenue by forcing cancer patients to pay for hospital car parking is morally wrong.
"It is shameful that the sickest and most vulnerable people have to pay the most. Hospital car parking costs are often the final straw in a long line of extra costs faced by cancer patients."
Michael Summers, chairman of the Patients Association, said: "We take the view that this is exploitation of patients who are very vulnerable.
"Some of these patients are visiting hospital several times in a week for treatment and a lot of patients' families are also visiting several times.
"Those who are genuinely attending for treatment ought not to be charged."