Labour peer Lord Janner has been ordered to appear at Westminster Magistrates' Court even though a doctor claimed his demenita meant he would not understand proceedings.
Doctors said the 87-year-old former MP for Leicester would find court "perplexing and bewildering", but chief magistrate Howard Riddle said he was fit to face charges.
He is accused of committing 22 sex offences in the 1960s, 1970s and 1980s and faced the unusual proceeding of being tried by the facts of the case in his absence.
The decision came after Dr James Warner told the court he had no doubt that Lord Janner has dementia "and that it is severe".
Questioned by doctors ahead of the hearing, Lord Janner became angry and irritated and tried to leave the room, the court heard, and that his condition was impacting his "day to day life".
But Riddle said the issue was not if Lord Janner understood the charges or proceedings but if he could physically appear in court for "less than one minute". The case has been adjourned for a week.