The Justice Department asked a federal judge this week to seal documents that might otherwise aid parents in lawsuits against the maker of a mercury-based vaccine preservative called thimerosal, which the parents claim caused their children's autism. The department has the right to make the request, but if the court grants it, parents could be prevented from getting evidence that might prove their claims. The court should refuse.

Courts occupy a borderland between the private and the public. In resolving disputes, they gain control of information that litigants wish to keep private. Some of this information deserves privacy, like trade secrets or details of a divorce. But information that alerts the public to danger or that might help prove responsibility for injuries should be publicly available once it is filed in court.

Similarly, a judge should not suppress information that enables the public to evaluate the performance of the courts, government officials, the electoral process and powerful private organizations. A federal appeals court was correct to unseal a letter that prosecutors had submitted to a trial judge last May in support of a lenient sentence for a political contributor who had aided their investigation of Senator Robert G. Torricelli of New Jersey. The letter, which contained evidence supporting the contributor's claims that he had given Mr. Torricelli thousands of dollars in cash and gifts, was initially sealed at the request of the prosecutors and the senator, who was seeking a second term. Within days after the court unsealed the letter, Mr. Torricelli was forced to withdraw from the race.

This is how things are supposed to work. In 1978, the Supreme Court noted that it was ''clear that the courts of this country recognize a general right to inspect and copy . . . judicial records and documents.'' Yet judges often seal records and order litigants to conceal what they may have learned in discovery before trial. By doing so, judges give the parties protection from public scrutiny that they could not get at a trial in open court. This benefits defendants by shielding possible misconduct. Plaintiffs benefit, too, because the prospect of court-imposed secrecy makes it more likely that defendants will offer more generous settlements. And courts benefit because cases are resolved more quickly.

Of course, the public pays for secrecy by losing the information that the trial would have revealed. Worse, even the request for sealing documents occurs in secret.

There is evidence that court-ordered secrecy is increasing, especially in defective products cases. A study in Dallas County, Tex., found that between 1920 and 1980, only 80 cases had sealed records. Between 1980 and 1987, the study found 200 sealed cases.

Not all judges comply with the wishes of the litigants. This week, Constance M. Sweeney, a state judge in Massachusetts, rejected the Boston archdiocese's motion to suppress 11,000 documents concerning the church's responses when priests were accused of sexually abusing children. The archdiocese had been ordered to give the documents to a lawyer representing four men suing the church, but it wished to keep them from public view while it sought to have the case thrown out. But Judge Sweeney properly refused to hide documents that contained information about a scandal involving serious harm and a powerful institution.

Defenders of court-imposed secrecy argue that it encourages settlements and avoids the costs of a trial. Some even argue that the courts' only job in private disputes is to help the parties resolve their differences and that the public interest enters the equation hardly at all. But when, as with thimerosal, a court is asked to suppress information that might help vindicate legal claims, or that reveals a continuing public danger or unethical behavior by powerful people or institutions, secrecy is intolerable. The harm is made worse when a judge, a public official, is asked to use public power to inflict it.