Three Illinois hospitals got some bad news last week: A big tax bill's coming. The Illinois Department of Revenue took away the property-tax exemption enjoyed by the three not-for-profit health centers: Northwestern Memorial Hospital's Prentice Women's Hospital, Edward Hospital in Naperville and Decatur Memorial Hospital in Decatur. The revenue department says its decision was based on an Illinois Supreme Court ruling last year that Provena Covenant Medical Center in Urbana hadn't provided enough charitable care to qualify for a tax exemption. Provena -- like other not-for-profit hospitals in Illinois -- was tax-exempt by law. Not-for-profit hospitals don't pay taxes and, in return, they're expected to provide some free or reduced-rate health care to people who can't pay for the care. But the law doesn't say exactly how much care the hospitals must provide.