President Trump. (Jabin Botsford/The Washington Post)

WHITE HOUSE press secretary Sarah Huckabee Sanders said Thursday that President Trump will submit to a full physical exam early next year and release the results. The announcement was meant to quiet speculations about Mr. Trump’s health after he seemed to slur his words in a speech the previous day. If, and this is a big if, Ms. Sanders’s announcement is more than a rhetorical maneuver to end the questioning — if the president actually follows through on his latest pledge to release information of public concern — it will be a welcome turn toward transparency for a man who has disrespected Americans’ interest in disclosure since he entered politics.

Yet there is a matter of presidential openness that is perhaps even more urgent. Congress is on the verge of passing the most significant tax bill in a generation. Nonpartisan experts report that the GOP plan would add $1 trillion or more to the debt to pay for a plan that would mostly benefit the rich. Mr. Trump has been unusually involved in determining the details of the plan. Yet the public can only guess at how he and his family would benefit from the bill, because he has broken with decades of bipartisan practice — and has flouted a direct campaign promise — by refusing to release any of his tax returns, past or present.

Left to speculate about Mr. Trump’s possible conflicts of interest, experts have identified provision after provision seemingly tailor-made to help the Trumps. Corporate tax breaks would disappear for many industries — but not for real estate. Income from businesses set up as “pass-throughs” would be taxed less, and the Trump Organization extensively uses the pass-through business design. Mr. Trump’s children would no doubt benefit from an eliminated or significantly curtailed estate tax.

Exactly how the estate tax would change is just one of the many details Congress must settle before passing a final bill — details the president could push in one direction or another. For example, the House version of the bill would eliminate the individual alternative minimum tax, which ensures that upper-end taxpayers cannot use tax breaks to zero-out their tax bills. By contrast, the Senate bill would leave the AMT in place in a modified form. A leaked copy of his 2005 tax return shows Mr. Trump paid $31 million because of the AMT that year. The president would appear to have a large financial stake in which version Congress chooses.

With these speculations swirling, Mr. Trump has claimed, without offering any evidence, that the tax bill would in fact cost him, not benefit him. That seems unlikely. But if it is so, all the more reason to let Americans see for themselves. Congress should insist on it before taking a final vote.