NEW YORK (CNNMoney.com) – It's not just the unemployed going without health insurance. Faced with soaring premiums in company-provided plans, millions of employed Americans are opting out.

A study released by the Robert Wood Johnson Foundation on Thursday shows that, between 1998 and 2003, the percentage of private-sector workers who enrolled in their employer's health insurance plan fell by 5 percentage points (or 3 million workers) to 80.3 percent.

The study looked at 40 states and found declines in 25 of them.

Pam Dickson, deputy director of the foundation's healthcare group, acknowledges that some of those 3 million additional workers who declined coverage might have signed on to a spouse's plan. But given that the number of plan-eligible employees at companies that provide health insurance remained about the same between 1998 and 2003, and given the rising number of uninsured in the country, she did not think that would account for the majority.

Cost is the No. 1 deterrent cited by the uninsured when asked why they don't have healthcare coverage. And it's the No. 1 reason for 55 percent of uninsured individuals who come from working families with access to an employer-sponsored insurance plan.

During the 5-year period studied, for example, the premium for individual coverage in employer-provided plans rose 42 percent to $3,481.

While the cost of the premium has gone up for both workers and employers, the portion of the premium paid by each remained fairly constant during the five-year period. Employers continued to pay the lion's share (82 percent of a worker's premium cost).

The Robert Wood Johnson Foundation report notes that the decline in insurance-eligible workers who enroll in their employers' plans can have a big impact not only on federal public programs like Medicaid, but also on state and local coffers, especially when combined with workers who are uninsured because their companies – smaller ones, in particular – are having a hard time affording health insurance plans.

"Efforts to constrain costs should be considered to ease the burden of coverage and access," the report's authors wrote.

There have been a number of ideas proposed in recent years, which proponents say could help curb healthcare costs.

The healthcare savings account (HSA) is a favorite of President Bush. HSAs are accounts to which you and your employer may make tax-free contributions up to a cap. Money in the account may be used to pay for eligible out-of-pocket medical expenses that you would incur to meet your health insurance plan deductible.

To open an HSA, you would have to sign up for a high-deductible plan. The premiums are lower, and consumers, HSA supporters say, are likely to be more cost-conscious in how they spend their healthcare money - for instance opting for generic drugs instead of more expensive brand-name prescriptions.

That could increase competition among medical care providers and drug manufacturers, supporters say. But critics contend they may discourage people from getting the care they need because of the expense and might not reduce healthcare costs much since those with chronic illnesses account for much of healthcare spending and won't have incentive to reduce their spending, since they need ongoing care.

Another proposal, put forth by the president's tax reform panel last year, is to curb the amount of tax-free health insurance contributions your employer makes on your behalf. Anything paid above the threshold would be treated as taxable income to you.

Proponents say the move could help curb insurance costs since employees might be less inclined to pay for expensive policies if they know they're going to be taxed on some portion of their employer's contribution.

In both these cases, however, not only is the jury still out, it hasn't been selected yet. Only 3 million people have signed up for HSAs, and serious debate on tax reform has been tabled for now.