User blog comment:KidVegeta/Canonicity/@comment-3097771-20101130083208

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Return of Cooler is not canon. There are several reasons for this. For one, this movie has to take place at least after the android sagas (Vegeta is Super Saiyan, and Dende is the guardian of Earth). However, during this brief time, none of the Z fighters would have time to go to New Namek, due to Cell. Also, Goku never powered down from Super Saiyan, during this time period (he did in the movie), and Gohan could turn Super Saiyan, which he was unable to in the movie.

Final Ruling: Canon