This year, the federal government ordered hospitals to begin publishing a prized secret: a complete list of the prices they negotiate with private insurers.

The insurers’ trade association had called the rule unconstitutional and said it would “undermine competitive negotiations.” Four hospital associations jointly sued the government to block it, and appealed when they lost.

They lost again, and seven months later, many hospitals are simply ignoring the requirement and posting nothing.

But data from the hospitals that have complied hints at why the powerful industries wanted this information to remain hidden.

It shows hospitals are charging patients wildly different amounts for the same basic services: procedures as simple as an X-ray or a pregnancy test.

And it provides numerous examples of major health insurers — some of the world’s largest companies, with billions in annual profits — negotiating surprisingly unfavorable rates for their customers. In many cases, insured patients are getting prices that are higher than they would if they pretended to have no coverage at all.

Until now, consumers had no way to know before they got the bill what prices they and their insurers would be paying. Some insurance companies have refused to provide the information when asked by patients and the employers that hired the companies to provide coverage.

This secrecy has allowed hospitals to tell patients that they are getting “steep” discounts, while still charging them many times what a public program like Medicare is willing to pay.

And it has left insurers with little incentive to negotiate well.