Gregory Katsas testifies before the Senate Judiciary Committee during his confirmation hearing to be a judge at the U.S. Court of Appeals for the District of Columbia Circuit, on Tuesday, Oct. 17, 2017.

If former Deputy White House Counsel Gregory Katsas had first-day jitters at his debut oral argument Monday, the newest judge at the U.S. Court of Appeals for the D.C. Circuit didn’t show it.

Katsas, nominated by President Donald Trump last September and appointed to the court in December, did not hold back when it came to questioning attorneys on both sides of the two cases he heard Monday morning. Katsas remained engaged throughout the roughly two-and-a- half-hour-long hearing, asking pointed questions about factual records and legal theories.

Katsas heard the arguments alongside two other judges who’ve been in the news recently: Chief Judge Merrick Garland, who was nominated by President Barack Obama for the U.S. Supreme Court in 2016 but denied a hearing by the Senate majority leader, and Judge Brett Kavanaugh, whose name appears on Trump’s latest list of potential Supreme Court nominees.

There were also some big names sitting in the audience Monday. Chad Readler, the chief of Department of Justice’s Civil Division, attended, as did Deputy Assistant Attorney General Hashim Mooppan. Katsas, Mooppan and Readler all worked at Jones Day before joining the Trump administration.

Monday’s cases, which focused on disability discrimination and Medicare payments, are by no means the most controversial to reach the court, which in recent months has decided major cases involving the constitutionality of the Consumer Financial Protection Board and the rights of undocumented minor immigrants to obtain abortions. Katsas, however, appeared to give complete attention to the advocates before him Monday, often making eye contact and nodding his head in what seemed to be agreement or understanding.

One case involved a disability discrimination lawsuit brought against the Department of Housing and Urban Development by an employee. Katsas questioned the appellant’s lawyer, Charles Elliot Wagner, about how many hours of work his client missed and the record of claims raised in proceedings within the agency.

When the government’s lawyer, Josh Kolsky, took the stand, he faced intense questioning from Katsas. The judge said he found it “troubling” that the government counted all of the employee’s 1,800 missed hours of work as showing she was not qualified for her job, instead of subtracting the time she took for annual and sick leave. Still, he conceded that the number of hours she took for approved leave was likely a “relatively small number.”

In the other case, more than 200 hospitals challenged a 2013 Department of Health and Human Services regulation. The hospitals argued HHS used faulty data in the 1980s that helped determine Medicare payment rates used today, and that the 2013 regulation illegally bars them from referencing the use of the faulty data in their challenges to reimbursement rates.

The questioning sparked some playful back-and-forth between the judges. Katsas, questioning Jackson Walker partner Edgar Morrison, asked about whether HHS was acting reasonably, given that it would be difficult for the agency to reinvestigate the data it used decades ago. He added that the alleged error in the data was “relatively small.”

“Well, it’s hundreds of millions of dollars,” Kavanaugh said, jumping in quickly. Katsas smiled, asked a clarifying question, and moved on.

Katsas also pressed the government’s lawyer, Melissa Patterson, on whether the text of the regulation showed it applies to appeal proceedings, a central issue in the case and a main point in the government’s argument, in addition to reopening proceedings.

Patterson used language in the preamble of the regulation to support her argument that it applied to appeals. But Katsas disagreed, arguing “the only textual reference” to appeals in the regulation itself did not prove that it applied to both kinds of proceedings.

“The statutory and regulatory structure seems crystal clear that an appeal is one kind of thing, and a reopening is another kind of thing,” the judge said.  

Katsas’ next oral arguments are set for Feb. 22.