"It never would have dawned on me," she said. "Why would I ever think such a thing?"
Yet many in Hollywood were thinking just that. Even her best friends had heard the rumor from their parents, but no one dared tell her.
Jack Haley Jr., a pal from high school days, said: "Our whole group knew who her parents were--and we knew she did not know. But we thought her mother would tell her."
Her mother-- Loretta Young--hadn't.
Young herself was the other half of the rumor--that Judy, told all her life that she had been adopted by the actress, was really Young's child by Gable.
In her recently published autobiography, "Uncommon Knowledge," Lewis says Young and Gable met in 1935 on the set of "Call of the Wild" and had an affair during which she was conceived.
Young, 81, declined to confirm or deny her daughter's account.
"This rumor is a product of (a) bygone time. As I have in the past, I have chosen not to give it any further credence," Young said in a statement from her Palm Springs home.
"Do you have any idea how sad it makes me that she still feels she must state that her daughter . . . is a rumor?" Lewis said in a recent interview.
In the 1930s, every star's contract had a morals clause. Gable was married to someone else; Young was single and Catholic. Any suspicion of an affair, let alone a child born out of wedlock, would have meant canceled contracts and careers.
Lewis said her parents elected to deny that they had ever had a relationship or a child. Gable backed out of the situation, leaving Young to make the decisions.
She grew up thinking she was adopted "in the home of my real mother," Lewis said. Whenever the child asked why her biological mother had given her up, or who her she was, Young was evasive, Lewis said.
Lewis writes that her nanny asked Young how to handle the girl's questions about her father. Young responded, according to the book: "You tell her that her father is dead. That's what he is . . . dead."
Lewis, now 58 with grandchildren of her own, said that even having her own family didn't erase the pain unwittingly inflicted by a mother so remote and relentless in her attempt to keep her child's heritage a secret.
"I was literally fatherless. It was as if I never had a father," she said, expressing wonder even now that something so important could have been withheld.
"I always felt half a person, never whole. That's one reason I had to write the book, to claim both my parents."
Lewis said she interviewed former nannies, priests, doctors, relatives, family friends, childhood pals and anyone else who participated in a life so painful that she had blacked out huge chunks of it. Through intensive psychotherapy in the past few years, she said, she has been able to retrieve early memories and establish enough self-confidence to "claim my parents."