It doesn't really matter what the US ratings are like for Lost, as it's the final season and doesn't need to fight for renewal, but for anyone who cares: the two-part premiere attracted a very healthy 12.09 million viewers for ABC, the third episode fell to 11m the following week, and the recent fourth episode lured 9.7m. So, viewership has been falling (although it's worth remembering that episode 4 was up against the Winter Olympics), possibly as curious audiences realize they don't have a hope in hell of understand what's going on at this late stage.
