The estimate, prepared at the request of Senate Democrats, is based on the partial repeal bill Republicans sent to President Obama's desk in 2015. Obama vetoed the measure, and Congress was not able to override it.
The office also estimated that individual health insurance premiums would increase by 20 to 25 percent in the first year of a repeal and would hit 50 percent after the elimination of the Affordable Care Act's Medicaid expansion.
Democrats seized on the new figures Tuesday, after rallies around the country opposing GOP plans to repeal and replace "Obamacare."
"The CBO's nonpartisan report shows that Republicans' plan to repeal the Affordable Care Act will be nothing less than a nightmare for the American people," House Minority Leader Nancy Pelosi, D-Calif., said in a statement.
“This projection is meaningless, as it takes into account no measures to replace the law nor actions that the incoming administration will take to revitalize the individual market that has been decimated by 'Obamacare,'" Ryan spokeswoman AshLee Strong said in a statement.
Republicans agree on repealing and replacing "Obamacare" — in conjunction with executive actions and administrative rules to ease any transition — but are still divided on the timeline and details of any replacement effort.
Ryan and Vice President–elect Mike Pence have said "Obamacare" will be replaced within the first 100 days of the administration. Republicans also hope to hold a repeal vote as early as next month.
Trump told The Washington Post his plan is "very much formulated down to the final strokes" and will be put forward after his pick for health and human services secretary, Rep. Tom Price, R-Ga., is confirmed.
Aides to Republican leaders in the House and Senate have yet to see any details of Trump's plan.
Republicans in the House and Senate adopted a budget measure last week to begin the "Obamacare" repeal process, directing the committees of jurisdiction to start crafting repeal language.
Trump also told the Post, "We're going to have insurance for everybody," closer to "universal coverage" than the "universal access" Hill Republicans say their plan will offer.
Still, Republicans are downplaying any tension in their plans for health care.
"We are on the same page," Ryan said Monday in an interview with Wisconsin news station WISC-TV. "We are all working on this together, working hand in glove with the new administration."
ABC News' Jonathan Karl contributed to this report.