The east wing was designed by Lloyd Morgan and Eugene Meroni and constructed in two stages between 1953 and 1960. The east wing is also referred to as One Madison Avenue. It replaced another building on the site, which was built in phases from 1893 to 1905, and which was also designed by LeBrun's firm. When the current east wing was built, the 700-foot tower was extensively renovated as well. In 2020, work started on an addition to the east wing, which will be designed by Kohn Pedersen Fox and be completed in 2023 or 2024.