Little Paige Furner's condition means she has been locked in a world of her own since she was born.
The three-year-old, of Lister Road, Braintree, has struggled to speak and had never called parents Karena and Jerry 'mum and dad'.
But now Paige is making amazing progress after a controversial treatment, using the hormone secretin, achieved startling results.
And there were tears of joy on Thursday when she finally spoke the words every parent wants to hear.
Stunned Mrs Furner, 37, said: "Paige walked into the room and said 'Hello Mummy'. It was enough to make me cry - I was so happy."
The toddler is receiving injections of hormones taken from pigs' stomachs.
Big-hearted folk raised thousands of pounds to pay for the ground-breaking treatment at a top London clinic because it is not available on the NHS.
Before Paige started the course of injections she was terrified of loud noises and would have screaming fits. The youngster - who has a brother, Jordan, aged five - even refused to wear a coat.
Mrs Furner said: "It's not a miracle cure. Things won't happen overnight but she is already much calmer. She never used to say goodbye and would cry when someone said it to her. But now she will and gives everyone a kiss.
"Over Christmas she has been amazing. She tried to say Father Christmas and can sing bits of Jingle Bells."
The family spent months agonising over whether to use hormones to help their daughter.
Car salesman Mr Furner, 48, and his wife took the gamble even though the long-term side-effects are still unknown.
Paige will have just three injections before they decide whether to continue with the treatment.
"We'll weigh up what Paige has gained against any long-term effects," Mrs Furner said.
"We are just so pleased we had the opportunity to do it. We thought about it for months. The doctors were saying 'don't do it', but you have to do something.
"It's not a wonder cure. It has not cured her of autism but the results we are getting are really important.
"I have no doubts we made the right decision to do it and give Paige every chance we can."
Paige and her family
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