Increasingly radical after the success of the October Revolution, he was one of the original members of the PCR (known as PCdR at the time) in 1921. Pătrășcanu, Elek Köblös, and Ana and Marcel Pauker were the representatives of the group to the 4th Comintern Congress in Moscow (November–December 1922). Back in Romania, Pătrășcanu was arrested and imprisoned at Jilava in 1924 (the year when the party was outlawed); he went on hunger strike until being relocated to a prison hospital.