As the film begins and ends in 1933, with Noodles hiding in an opium den from Syndicate hitmen, and the last shot of the movie is of Noodles in a smiling, opium-soaked high, the film can be interpreted as having been a drug-induced dream, with Noodles remembering his past and envisioning the future. In an interview by Noël Simsolo published in 1987, Leone confirms the validity of this interpretation, saying that the scenes set in the 1960s could be seen as an opium dream of Noodles.[7] In the DVD commentary for the film, film historian and critic Richard Schickel states that opium users often report vivid dreams and that these visions have a tendency to explore the user's past and future.[8]
http://en.wikipedia.org/wiki/Once_Upon_a_Time_in_America#Plot