Although the official title of the album is Chicago, it came to be retroactively known as Chicago II, keeping it in line with the succession of Roman numeral-titled albums that officially began with Chicago III in 1971.
While The Chicago Transit Authority was a success, Chicago is considered by many to be Chicago's breakthrough album, yielding a number of Top 40 hits, including "Make Me Smile" (#9), "Colour My World" (#7), and "25 or 6 to 4" (#4). The centerpiece of the album was the thirteen-minute song cycle "Ballet for a Girl in Buchannon". Guitarist Terry Kath also participated in an extended classically styled cycle of four pieces, three of which were co-written by the well-known, arranger, composer, and pianist Peter Matz. The politically outspoken Robert Lamm also tackles his qualms with "It Better End Soon", another modular piece. Peter Cetera, later to play a crucial role in the band's music, contributed his first song to Chicago and this album, "Where Do We Go From Here".
Released in January 1970 on Columbia Records, Chicago was an instant hit, reaching #4 in the US and #6 in the UK.
Columbia Records was very active in promoting its quadraphonic four-channel surround sound format in the mid-1970s, and nine of Chicago's first ten albums were made available in quad. The quad mix features elements not heard in the standard stereo mix, including additional guitar work from Kath in "25 Or 6 To 4" and a different vocal take from Lamm in "Wake Up Sunshine," the latter of which reveals a different lyric in the song's last line.
In 2002, Chicago was remastered and reissued on one CD by Rhino Records with the single versions of "Make Me Smile" and "25 or 6 to 4" as bonus tracks.
Rhino released a DVD-Audio version of the album in 2003, featuring both Advanced Resolution Stereo and 5.1 surround sound mixes.