Trade unions are often identified in frame of media discourse as stakeholders reluctant to change in flexible work systems. The results of the research methodology CRANET in large private companies in Warsaw and Mazovia region suggest that at the company level does not exist the trade union effective opposition in an enhanced form. In companies where unions exist, compared with companies where they are weak or not present at all, escalating phenomenon of employment flexibility is similar. The only exception is a contract for a fixed period: trade unions are likely to effectively oppose against the introduction of such contracts on a large scale at the company level.