1999–2003: Breakthrough years and mainstream success
In June 1999, Yoon Eun Hye replaced Lee Gai to create a stable line-up. The single "Get Up" was released. For the first time, the group reached number one on the South Korean music pop charts. Another single, "Killer", was also number one and received the Top Excellency Award in the Seoul Music Awards in 1999. The third single, "Missing You", was also included on their third album Come Come Come Baby (1999).
In the years that followed, Baby VOX made a number of appearances in variety shows. After the success of the third album, the group toured internationally, including in China, Japan and other Southeast Asian countries. The fourth album, Why, included the singles "Why" and "배신" (Betrayal), and the group hosted the television program Beautiful Sunday-Cruise to the Korea Strait. The group's fifth album, Boyish Story, was released in 2001 and included the singles "Game Over", "인형" (Doll) and "I Wish You are My Love".
The group released a compilation album with singles "우연" (Coincidence) and "Go". "Coincidence" was the group's first number one single in three years. "Coincidence" was re-released for the 2002 World Cup and gained additional popularity. The group held a concert in Mongolia in 2004, the first South Korean idol band to do so. The group also performed in Pyongyang, North Korea in 2003, and was the second girl group to perform in North Korea.
