She obtained several number one hits on the Oricon charts in 1984: "Nagisa no Haikara Ningyo / Kaze no Magical", "Yamato Nadeshiko Shichi Henge" and "The Stardust Memory", the latter holding the top spot over the end of 1984 and the beginning of 1985, and thereafter established herself as one of Japan's most popular pop idols, alongside rivals Seiko Matsuda and Akina Nakamori.

Koizumi went on to release another single (優しい雨) which also sold over a million copies in 1993.

Koizumi had singles reach the Top Ten for 12 consecutive years between 1983 and 1994, a female solo artist record, until this was broken by Namie Amuro.

She had her first hit single in 18 years when her version of the song "Shiosai no Memorī" from the 2013 NHK morning television drama Amachan, in which Koizumi also acted, reached number 2 on the Oricon Singles Chart.

[5][6] With that, Koizumi made her 6th appearance (as a special guest performer) in the annual Red and White Song Festival, having previously participated from 1984 to 1988.