Mammoth Uncensored Word List:
16 words that are 20 letters long beginning with u in the mammoth uncensored word list.

ultracentrifugations ultramicroscopically ultrarevolutionaries ultrasegregationists ultratraditionalists uncharacteristically uncompromisingnesses unconscionablenesses underrepresentations unintelligiblenesses unquinquagintilliard unquinquagintillions unrepresentativeness unsatisfactorinesses unviginticentillions ureterosigmoidostomy