The trial of Josef Fritzl commenced on 16 March, 2009 in the town of Sankt Pölten, presided over by Judge Andrea Humer.

On day one, Fritzl entered the courtroom attempting to hide his face from cameras behind a blue folder, which he was entitled to do under Austrian law. After opening comments, all journalists and spectators were asked to leave the courtroom, whereupon Fritzl lowered his binder. Fritzl pleaded guilty to all charges with the exception of the murder and grievous assault by threatening to gas his captives if they disobeyed him. The closed-door trial is expected to last five days, with live and videotaped testimony on the first four days and the verdict, delivered with journalists and spectators present, on the fifth.
In opening remarks, Rudolf Mayer, defending, appealed to the jury to be objective and not swayed by emotions. He insisted Fritzl was "not a monster," noting that Fritzl had brought a Christmas tree down to his captives in the cellar during the holiday season.
Christiane Burkheiser, prosecuting her first case since being appointed Chief Prosecutor, is pressing for life imprisonment in an institution for the criminally insane. She demonstrated for jurors the low height of the ceiling in the cellar dungeon by making a mark on the door to the courtroom at 1m 74cm (5ft 8.5in), and described the cellar as "damp and mouldy," passing around a box of musty objects taken from the cellar, whose smell made jurors flinch.
On the first day of testimony, jurors began watching an 11-hour testimony recorded by Elisabeth in sessions with police and psychologists in July 2008. The tape is said to be so "harrowing" that the eight jurors will not have to watch more than two hours at a time. Four replacement jurors will be on standby to take over from any member who cannot stomach the evidence. Josef's wife, Rosemarie, and Elisabeth's children have refused to testify.
Court officials refused to confirm or deny claims made by the Kurier newspaper that Elisabeth had been present as a visitor in court but intimated that she had been present on the second day by insisting she had definitely not been in court on the third day. It can be assumed that she was in court – together with her brother Harald and another brother or sister. The same day it was announced that Fritzl, in response to his daughter's video testimony, had changed his pleas to guilty on all charges.