in the book, Kay made contant inquires about Michael which eventually reunited her with him after his exile to Sicily. in the flick, he returns for her.

in the novel, Vito Corleone states to Signora Colomba's landlord that he is not too fond of animals, yet in the movie, we see him quite fond of his cat (could be a Brando preference)

in the book, Michael had two sons. in the flick, he has a son and a daughter.

in the book, none of the Don's grandchildren have names, as opposed to the movie.