After years of stuttering attempts at broadening its appeal to girls, Lego may have found its answer to bridging the gender gap.

Introduced in 2012, the “Lego Friends” line of bricks from the Danish toymaker has fast become a hit with girls, and is proving to be a winner. Lego told the Wall Street Journal the share of girls among Lego players, which hovered below 10% in the U.S. before the arrival of Lego Friends, has increased sharply since.

“We had made so much research and so much testing on girls that we were never in doubt about the product itself,” Lego senior design manager Benedikte Schinkel Stamp told the Journal.

Lego’s success among girls can be seen in the market for girls’ construction toys in the U.S. and the larger European countries, which tripled to $900 million in 2014 from $300 million in 2011, according to research firm NPD Group. This increase is believed to be due to the entry of Lego Friends.

In Lego’s published financials for last year, its “Girls Building Sets” category saw significant double-digit growth of 28% on year-end sales from the previous year. The company said this was largely driven by sales in its Lego Friends and Lego brand Disney Princess line.

Long thought of as a boy’s toy, Lego had tried in previous years to launch lines targeted at girls, with little success, reported the Journal. Scala, a line of buildable jewellery, was launched in 1979 and discontinued a year later. In 1992, a pink and pastel-colored set called Paradisa was created, but was halted in 1997. It took years of research to find out that girls enjoyed building just as much as boys and really just differed in what they wanted to build, along with the colors in which they favored their sets.

“It’s a real construction toy,” Lego design director Rosario Costa told the Journal about Lego Friends. “Not dumbing it down.”

The news caps a year of great news for Lego, which in September, became the world’s biggest toymaker by sales, surpassing Mattel mat and Hasbro has . Demand for Lego sets during this year’s holiday season were so high that the company intimated there could be a shortage of its products as it struggled to keep pace with customers.