Valentina Petrillo has become the first transgender athlete at the Paralympics - and qualified for the women's 400m semi-finals on Monday.
The 51-year-old was still competing as a male just six years ago but has been allowed to compete in the Games in Paris.
Petrillo came second in her heat and beat a Chinese runner who is 18 years her junior.
Venezuela's Alejandra Paola Perez Lopez was the only person to clock a faster time at 56.97 seconds.
Valentina Petrillo has become the first transgender athlete at the Paralympics - and qualified for the women's 400m semi-final on Monday.
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The World Para Athletic (WPA) insist that Petrillo's testosterone levels mean she is fit to race against female athletes.
Though partially sighted, the Italian ran unaided.
Two other runners, from Venezuela and China, needed a guide during the race.
Petrillo ran her heat in a time of 58.35, with Shen Yaqin missing out on qualification in third.
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There has been plenty of backlash about the 51-year-old's participation in the Games, especially due to the fact she's competing against biological women.
One of her rivals, speaking before the race, said last month that she was concerned about the Italian having an unfair advantage.
"Basically, everyone should live in everyday life the way they feel comfortable," Katrin Mueller-Rottgardt told Bild.
"But I find it difficult in competitive sports.
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"She (Petrillo) has lived and trained as a man for a long time, so there is a possibility that the physical requirements are different to those of someone who was born a woman.
"This could give her an advantage."
Madrid-based lawyer Irene Aguiar also raised concerns.
"Our Spanish athlete Melani Berges has lost the chance to qualify for the Paralympics," she said before the Games. "The reason is the participation of the man Fabrizio 'Valentina' Petrillo, who made it to the final instead of her. That is unfair."
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Valentina Petrillo came second in the heats of the 400m at the Paris Paralympics
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Speaking before the Paralympics, Petrillo opened up on how transitioning had changed her.
"My metabolism has changed," she told the BBC.
"I'm not the energetic person I was. In the first months of transition I put on 10kg. I can't eat the way I did before, I became anaemic, my haemoglobin is low, I'm always cold, I don't have the same physical strength, my sleep isn't what it was, I have mood swings.
"I'm not the same as before."
Valentina Petrillo will continue to compete at the Paris Paralympics
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Petrillo also said her times had become slower, too.
"As a sportsperson, to accept that you won't go as fast as before is difficult," the 51-year-old added.
"I had to accept this compromise, because it is a compromise, for my happiness."