David Mayhew (1991) argued that divided government does not lead to gridlock; partisan rivals find ways to strike deals. In parliamentary systems, however, the stakes are higher, because government survival is partly a function of legislative effectiveness. If a parliamentary system is strongly bicameral, a government could face an opposition majority in the upper house with the means and motivation to block its legislative agenda and oblige it to resign. In this paper, we examine data from the Japanese Diet between 1989 and 2013, a period of frequent non-government upper house majorities. Have Japan's “Twisted Diets” succumbed to gridlock? We show that while governments adapt smoothly to the mere absence of upper house control, they are severely hampered when the upper house is controlled by the Opposition. They propose fewer laws and suffer more changes to and failures of the legislative proposals they do submit.