During Italy's ‘two red years’ (1919–20), left-wing catholics challenged the authority of the church and the landowners in large areas of northern Italy. Calling themselves the estremisti (the extremists), left catholic unions organized peasants and workers in land and farm occupations and encouraged a series of radical strikes. Left catholic leaders became national figures, in particular Guido Miglioli at Cremona and Romano Cocchi at Bergamo. This article examines these innovative struggles and their troubled relationship with the traditional socialist Italian left during this turbulent period. No alliances were formed between the estremisti and the ‘red’ unions until 1921–4, when fascism was already rampant and the revolutionary wave had already subsided. The article analyses why alliances were not built earlier, and why the socialists were so hostile towards the catholic left. Both the theory and the practice of the traditional left prevented any positive appraisal of the estremisti. In addition, there are detailed accounts of the extraordinary mass movements inspired by Cocchi and Miglioli in some of the richest and most staunchly catholic regions of northern Italy.