When asked by the presiding Judge Sebastian Buehrmann whether the 100 allegations of abusing patients to death were largely true, Hoegel replied, "yes".
He added: "What I have admitted took place".
A German nurse named Niels Hoegel who was convicted of killing six patients by injecting them with deadly drugs has admitted he may have actually killed as many as 100 patients.
Hoegel is accused of carrying out the murders between February 2000 and June 2005 in hospitals in the cities of Oldenburg and Delmenhorst.
"We will strive to seek the truth with all our strength".
He was found guilty of murder and attempted murder of five other victims and given the maximum sentence of 15 years.
The main building of the former Klinikum Delmenhorst hospital (now Josef-Hospital), where former male nurse Niels H. allegedly killed dozens of patients.
According to reports, the trial began with a minute of silence in memory of the victims, whose ages ranged from 34 to 96.
Murders were committed in the period from 1999 to 2005.
Despite suspicion about the mounting deaths on Hoegel's watch, the hospital did not open an investigation.
Hoegel appears to have followed a similar procedure each time, first injecting a medication that triggered cardiac arrest, and then stepping in to resuscitate them.
The Oldenburg state court is conducting the trial at a courtroom set up in a conference centre, a venue chosen to accommodate a large number of co-plaintiffs as well as public interest in the proceedings.
Hoegel is accused of giving his victims various non-prescribed drugs, in an attempt to show off his resuscitation skills to colleagues and fight off boredom.
Christian Marbach, whose grandfather was killed by Hoegel, described him to the Heidelberg Heraut newspaper as a "sad guy who gave himself God's powers".
Investigators say the final toll could top 200 but fear they might never know for sure because the bodies of many potential victims were cremated.
The 41-year-old was jailed for murdering two patients in 2015, but investigators later discovered the extent of his crimes were likely to be much greater than previously thought.
Additional convictions could make it harder for Hoegel to get parole. Investigators are building a case against former staff at both facilities where Hoegel said he killed his patients.