The Tacoma-Pierce County Board of Health adopted the fluoridation rule in October 2002, saying it would save millions of dollars in medical costs and prevent children from having dental problems.

A group of citizens, cities and private water companies challenged the rule in court. They argued that the health board had overreached its power, that the fluoridation order amounted to an illegal tax, and that it would force unwanted medical treatment on some citizens.

The Supreme Court ruled that state law specifically gives water districts - not local health boards - the authority to decide whether to fluoridate their water.

"The board's resolution irreconcilably conflicts with the authority granted to water districts," Justice Charles Johnson wrote in his majority opinion.

Six justices sided with Johnson, and three dissented.

Justice Faith Ireland wrote in her dissent that the board's fluoridation order was "a proper exercise of its police power."

State law, she wrote, gives a local health board "supervision over all matters pertaining to the preservation of the life and health of the people within its jurisdiction," and that the Tacoma-Pierce board based its decision on evidence that dental disease is a serious health problem, especially for children.

Federico Cruz, director of the Tacoma-Pierce County Health Department, has said that about 70,000 children in the county don't have good access to dental care. Nearly a quarter of those children have significant dental diseases.

The case is No. 73734-7, Parkland Light & Water Company et al v. Tacoma-Pierce County Board of Health et al.