House Speaker Paul Ryan is retiring at the end of the current Congress, in January, after taking over as speaker in October 2015.

Ryan said one of his biggest disappointments as speaker was not addressing the growing amount of federal debt.

As speaker, Ryan oversaw the passage of legislation expected to add trillions to the deficit.

In an interview on Thursday, Rep. Paul Ryan said one of the biggest disappointments from his term as House speaker was his inability to address the growing mountain of federal debt.

"On healthcare itself and debt and deficits, it's the one that got away," Ryan said at a Washington Post event.

As speaker, Ryan helped oversee a rise in spending that is already adding to the deficit. Under his tenure, the amount of outstanding federal debt grew by just over $2 trillion, and the federal deficit expanded from $438 billion in the 2015 fiscal year to $779 billion in the 2018 fiscal year.

And it's projected to get even more pronounced from here. The Congressional Budget Office estimates that the deficit in the 2019 fiscal year — which began October 1 — will be $981 billion. For the 2020 fiscal year, the CBO projects that the deficit will eclipse $1 trillion for the first time since the depths of the financial crisis.

While there are plenty of reasons for the growing deficit, recent legislation spearheaded by Ryan has only added to it.

Ryan has had a reputation as a policy wonk, and he entered the speakership with goals to address rising spending on entitlement programs. During his tenure as speaker, however, the House did not embark on any significant effort on that front.

The nonpartisan Committee for a Responsible Federal Budget estimates that 46% of the deficit in the 2019 fiscal year, or $445 billion, will be derived from legislation passed since Trump took office. A little more than half of that amount comes from the GOP tax law — which Ryan has cited as one of his major accomplishments — with most of the rest coming from the bipartisan budget agreement.

In 2018, Ryan's last year in Congress, the US is expected to issue $1.3 trillion in new debt, up 146% from the 2017 amount and the most in one calendar year since 2010.

Read more: Trump is suddenly obsessed with the ballooning federal deficit after his signature legislative victories caused it to explode

The Tax Cuts and Jobs Act is also projected to add to the deficit and debt for years to come. The CBO expects the tax law to add about $1.5 trillion to the federal deficit through 2028, even after factoring in any increase in economic growth caused by the law.