Charles Cullen appeared in a Lehigh County courtroom to enter the plea, which was announced last month.
With his latest admission, Cullen has now pleaded guilty to giving fatal medication overdoses to 23 people and nonlethal overdoses to five others during his 16-year career.
The Lehigh County charges involved five people who died at St. Luke's Hospital in Fountain Hill, where Cullen worked between 2000 and 2002, and one at Lehigh Valley Hospital in Salisbury Township, where Cullen worked from 1998 to 2000.
Two more St. Luke's patients at the hospital got sick when Cullen gave them a medication overdose, and later died, but prosecutors said their deaths may have been from other causes. At Lehigh Valley Hospital, a patient received an overdose that was not fatal.
All of Cullen's victims were seriously ill, but not all were expected to die soon when he delivered his poison, prosecutors said.
Additional deaths are still under investigation and Cullen may face further charges in New Jersey.
Cullen has claimed to have killed 30 to 40 people. He agreed to help investigators identify his victims in exchange for a promise that they would not seek the death penalty.
In August 1997, Cullen was fired from Morristown Memorial Hospital in New Jersey for "poor performance," according to a spokeswoman for the hospital's parent company.
Cullen worked at St. Luke's Hospital in Bethlehem, Pa., from June 2000 to June 2002, and resigned amid allegations that he had at least twice hidden unopened heart and blood pressure medications in a safety bin for used needles, Lehigh County District Attorney James Martin said.
St. Luke's spokeswoman Susan Schantz said the hospital was never subsequently contacted by anyone checking Cullen's employment references. "Had we been asked, we would have recommended that he not be hired," she said.
Cullen had no record of complaints or any disciplinary actions in New Jersey since he obtained a nursing license in the state in 1987, according to Genene Morris, a spokeswoman for the Division of Consumer Affairs. Cullen was licensed to work in Pennsylvania since June 1994, according to the Pennsylvania Department of State. His license was in good standing, officials said.
Officials in Pennsylvania say they don't expect to bring anymore charges against him, but there are more deaths still under investigation in New Jersey.
Cullen will be sentenced in January. He asked the judge Wednesday to be excused from the sentencing hearing but the request was denied. Many of the victims' family members say they want to use the opportunity to confront Cullen.