Uma Lindström had reacted to finding out that the T-shirt she wanted could only be found in the boys section.
"I thought it was bad that it was just flowers and horses on the girls side, and on the boys side, it was The Simpsons and cool stuff," says the ten-year-old to Swedish Radio News.
The blog entry with Uma Lindstrom's open letter to H & M was put out last Friday and has been widely distributed online. Today there are more than 90,000 people who have read her letter.
Uma Lindström , who likes video games, wanted a T-shirt with Super Mario on it and saw that it was sold at H & M - but only in the boys department. She says to Swedish Radio News that the idea is wrong that girls should have horse motifs while boys have video game designs on their clothing. So, she wrote to H & M.
"It is unfair that someone gets to decide what is for girls and what is for boys," says Uma Lindström.
"Most of our customers appreciate our set-up in our stores. Of course, everyone is welcome to shop in all our departments," H&M wrote when contacted by Swedish Radio News.
It's not just H & M that divides their clothes after sex. And Uma Lindström says she will write letters to other clothing chains as well. "As this has become so well publicised I will write many more letters to other retailers."