Others are questioning whether some tax-exempt nonprofits, primarily universities and hospitals, have accumulated so much wealth that they should no longer be considered charities. In Massachusetts, where Harvard’s endowment has reached $35 billion in assets, legislators are weighing whether to impose a 2.5 percent annual assessment on universities with endowments of more than $1 billion.

The idea behind tax exemptions is that the organizations provide a public service or substantially reduce the burdens of government. Standards from property-tax exemptions are set by the states, while the federal exemption means charities are not taxed on their income.

Almost 88 percent of overall nonprofit revenues in 2005, the most recent year for which figures are available, came from fees for services, sales and sources other than charitable contributions, according to the National Center for Charitable Statistics. Nonprofit health care providers, day care centers and retirement homes, among others, are often difficult to distinguish from their tax-paying competitors.

“We’re all seeing the growth of revenue in this area we call earned income,” said Audrey R. Alvarado, executive director of the National Council of Nonprofit Associations, adding that the Minnesota court decision “is saying, ‘Wait a minute, charities are supposed to give things away for free.’ ”

“It goes to the core of how nonprofits are classified and defined,” she said, “and I think it is an example of the confusion in the public, and even among folks in the sector itself, about what a nonprofit is.”

Evelyn Brody, a professor at Chicago-Kent College of Law and an expert on nonprofits and property taxes, said that, in studying the issue in 2002 and revisiting it last year, she had seen an explosion of cases across the country in which charities were challenged to say why they deserve their property tax exemptions.

As universities charge high tuitions, and pay large salaries to administrators, they have become prime targets. For example, New London, Conn., assessed property taxes on a skating rink owned by Connecticut College.