Pelosi accused the nonpartisan budget analysts of always providing "the worst case scenario" on the costs of health care reforms and ignoring the savings associated with the proposal.
Congressional budget officials said Monday the health care bill would cost the government an estimated $1 trillion over the next decade and reduce the ranks of the uninsured by about one-third, or 16 million individuals. In a competing proposal, the cost equaled $1.6 trillion.
The CBO released its preliminary estimate as several congressional committees looked ahead to votes on legislation that President Obama has placed atop his list of domestic priorities.
"It's always been a source, yes I will say frustration, for many of us in Congress that the CBO will always give you the worst case scenario on one initiative and never ... any credit for anything that happens if you have early intervention, health care," Pelosi said in her weekly news conference on Capitol Hill.
"If you have prevention, if you have wellness ... you name any positive investment that we make, that we know reduces cost, brings money to the Treasury in the case of education but never scored positively by the CBO. Yes, it is frustrating," she said.
She added, "I hope we will see them say, 'This is what we see the cost of something. We have not accounted for the benefits' because they don't and they haven't and it should not be inferred from what they do that they have."
The CBO declined to comment.
In a letter to Sen. Edward Kennedy, D-Mass., Congressional Budget Office Director Douglas W. Elmendorf said the estimate was based on major provisions contained in an incomplete draft of the bill. He noted that "taking all of its provisions into account could change our assessment of the proposal's effects on the budget and insurance coverage rates though probably not by substantial amounts relative to the net costs already identified."
Meanwhile, Republicans continue to keep pressure on Pelosi to explain her accusation last month that CIA briefers misled her on Bush-era interrogation techniques. House Republicans are pressing for an investigation into the allegations.
FOX News' Mosheh Oinounou and The Associated Press contributed to this report.