After the disbandment of Z-1, Ueto was signed under the Pony Canyon sublabel, Flight Master. Ueto initially declined the offer but was eventually persuaded by her talent agency. Her debut single was scheduled for a May release, however, with Seiko Matsuda's daughter, Sayaka Kanda, also debuting around the same time, her management team decided to push back Ueto's single and "Pureness" was subsequently postponed to August. Out of the 600 songs they received, three were chosen for the single by Ueto and her team. "Pureness" was written and produced by T2ya. The look for the cover artwork and music video was based on Nao Tsurumoto, Ueto's breakthrough role on the TBS drama 3-nen B-gumi Kinpachi-sensei.
"Pureness" was released in two formats: the limited edition, which includes a DVD featuring the music video for "Pureness" and a special video message from Ueto, and standard CD-only edition. The cover art for the limited edition represents doubt, while the cover for the standard edition represents hope.
On August 28, 2002, Ueto held a special event to commemorate the release. The promotional event, held at Zepp Tokyo, attracted over 2,000 fans. Due to overwhelming fan demand, a second event took place at Zepp Osaka on September 15, 2002. Ueto performed "Pureness" for the first time on NHK's Pop Jam Summer Special. She also appeared on Utaban and Hey! Hey! Hey! Music Champ. She performed the song for the first time on live television on Music Station and was invited to perform at their televised year-end concert Music Station Super Live 2002.
"Pureness" debuted on the Oricon Daily Singles chart at number 5 on August 27, 2002 and climbed to number 1 on August 31, 2002. It peaked at number 4 on the Oricon Weekly Singles chart, with 46,140 copies sold in its first week. "Pureness" became the first debut single with no commercial tie-in by a female artist to enter the top five in three years, since Kyoko Fukada's "Saigo no Kajitsu" (1999), which was coincidentally released under the same record label and also debuted at number 4. The single charted for eight weeks and has sold a total of 96,350 copies.