This week, Shawne Kleckner, head of RightStufAnime/Nozomi Entertainment did a Reddit Ask Me Anything (AMA) about their Kickstarter for Aria. The subject of the 2004 anime adaptation of Naoki Urasawa's award-winning Monster came up. He downplayed the possibility of a release, explaining "this one is stuck in a licensing quagmire that isn't likely to be resolved anytime soon."
Along with a release of the manga, Viz released a box set of the first 15 of the series' 74 episodes in 2009, but dropped the project due to low sales.
They describe the series:
Everyone faces uncertainty at some point in their lives. Even a brilliant surgeon like Kenzo Tenma is no exception. But there’s no way he could have known that his decision to stop chasing professional success and instead concentrate on his oath to save peoples’ lives would result in the birth of an abomination. The questions of good and evil now take on a terrifyingly real dimension.
Years later, in Germany during the tumultuous post-reunification period, middle-aged childless couples are being killed one after another. The serial killer’s identity is known. The reasons why he kills are not. Dr. Tenma sets out on a journey to find the killer’s twin sister, who may hold some clues to solving the enigma of the “Monster.”
In 2013, Pacific Rim director Guillermo del Toro started developing a live-action adaptation for HBO with Steven Thompson (Doctor Who, Sherlock). Speaking at the fourth annual Hero Complex Film Festival, Del Toro said he has submitted a pilot script.
Later, in 2015, he gave an update to Latino-Review
No, it's out of HBO, and now we're going to take it to other places but it's not active active until we pitch it. We are going to other companies to pitch it and see if anyone wants to do it. We finished writing a couple of the episodes, and so we have a sampler. We're going to go and see what happens with that, but very likely that will happen somewhere next year.
Before that, New Line Cinema acquired the rights for an American live-action film adaptation, with Josh Olson (A History of Violence) hired to write the screenplay for a planned 2009 release.