Suggesting that 20 years in prison was not enough punishment for the crime, a federal judge on Friday rejected a plea agreement for a former hospital technician and drug user who admitted that she exposed hundreds of patients in her care to hepatitis C.

The judge, Robert E. Blackburn, said the agreement with the former hospital worker, Kristen D. Parker, inordinately restrained his discretion and did not take into account the views of victims, many of whom submitted anguished written statements. It is unusual, legal experts said, for a judge to reject a plea agreement.

Ms. Parker, 27, admitted to the police on videotape that while working at Rose Medical Center in Denver in 2008 and 2009, she stole pain-medication syringes from operating room trays, replacing them at times with needles she had already used to inject herself with heroin.

Seventeen Rose patients have so far been found to have a strain of hepatitis C linked through genetic sequencing to the strain in Ms. Parker’s blood, according to the Colorado Department of Public Health and Environment. Hepatitis C affects liver function and can have lifelong consequences.

Ms. Parker’s lawyer, Gregory C. Graf, said he had not consulted yet with his client but expected she would probably persist with her guilty plea, giving the judge discretion as to her sentence when the case reconvenes next month. Ms. Parker could also change her plea to not guilty and insist on a jury trial, or try to reach another plea agreement with prosecutors.

Judge Blackburn warned Ms. Parker in the brief hearing in Federal District Court, before a courtroom packed with former Rose patients and their families, that if she chose to continue with her guilty plea, the sentence could be stiffer.

“I may dispose the case less favorably,” Judge Blackburn said.

One legal expert who specializes in criminal law and sentencing, Douglas A. Berman, said judges are generally loath to throw out plea bargain agreements, partly because there is an assumption that victims have already been consulted.

But Professor Berman, who teaches criminal law at the Ohio State University law school, said the Federal Crime Victims’ Rights Act passed by Congress in 2004 opened up channels that in many cases bypass prosecutors, putting new pressure on judges.

A lawyer representing 13 of the Rose hepatitis patients, Hollynd Hoskins, also specifically argued to Judge Blackburn that the United States attorney’s office was not adequately consulting with her clients as required by the Victims Rights Act.

The United States attorney for Colorado, David M. Gaouette, in a statement on Friday, said, “The victim issues are vitally important in this case and we will continue to work closely with them to ensure their voices are heard.”

By KIRK JOHNSON