This paper defines the main objectives, stages, and the dynamics of the secret cooperation of the democratic Hungarian opposition, hostile to the Horthy regime, with the government of Czechoslovakia in 1919-1932. It focuses on the Prague's contacts with Hungary's Octobrists, social democrats (active both within the country and in exile) and liberals. The paper covers mostly the period of the so-called consolidation of the Horthy regime, carried out under the leadership of Prime Minister István Bethlen. Our research concludes that the struggle of the democratic opposition against the Horthy-Bethlen regime was consistently encouraged by Czechoslovak political and diplomatic circles. The collaboration between anti-Horthyist groups and Prague was particularly intense in 1919–1921 and in 1930–1931. Our study utilises hitherto unknown documents from archives in Prague and Budapest, to re-evaluate the causes of interwar tensions between Hungary and Czechoslovakia — beyond their disputes over borders and disagreements over the treatment of minorities.