The UK's second chamber is generally seen as a historical curiosity and, as Labour governments post-1997 have failed to introduce elected members, continues to be seen as ‘unreformed’. But a ‘first stage’ reform of the House of Lords in 1999 removed most of its hereditary members, leaving it almost wholly made up of appointed life peers. Although many at the time claimed that this would neuter the upper house, since most of those evicted were Conservatives, it instead seems to have had the opposite effect. This article examines whether the 1999 reform strengthened the House of Lords, how we can assess this and why it might have happened. The article concedes that measuring legislative influence is notoriously difficult. The analysis it constructs seeks to overcome this by using a range of indicators on two dimensions: the assertiveness of the House of Lords to use its powers, and the executive's responses to the chamber. This analysis indicates that the 1999 changes strengthened the Lords against the government and, in doing so, strengthened parliament as a whole. The most obvious consequence is that our understanding of British politics now needs revision. But if the Lords was strengthened by reform this also demonstrates a need to revise established comparative politics theories of bicameralism – particularly that of Arend Lijphart, which would have predicted the reverse. The recent developments in Britain validate George Tsebelis' approach of recognising ‘partisan veto players’ and the critical role of partisan balance in inter-cameral relations. But they also demonstrate that a more nuanced understanding of both legitimacy and legislative influence is needed in order to understand second chambers' (and indeed first chambers') de facto power.