Among the celebrities to enter the house last night were 'The X Factor' contestant Rylan Clark, former Liverpool footballer Neil Ruddock, reality TV couple Heidi Montag and Spencer Pratt, and jockey Frankie Dettori.
According to unofficial figures, 'Celebrity Big Brother' was watched by an average of 3.30 million viewers between 9pm and 10pm across the standard and high-definition Channel 5.
An additional 176,300 watched the show on Channel 5 +1.
At the beginning of last night's show, Clark and Dettori had to choose which of the celebrities would join them in the luxury part of the house, and which would have to live in the grimy cellar downstairs.
The five-minute peak audience of 4.33 million viewers was achieved at 10.10pm, when Page 3 girl Lacey Banghard and former ‘Coronation Street’ actor Sam Robertson entered the house.
The headline sponsor of the show is the bed shop Dreams and yesterday, Media Week revealed Very.co.uk had signed a click-to-buy partnership with Channel 5 around the show.
Last year's 'Celebrity Big Brother' launch had an overnight peak audience of 4.24 million viewers on 5 January 2012, while Denise Welch's victory in the final attracted an overnights peak audience of 3.8 million viewers.
Nick Bampton, commercial sales director at Channel 5, said: "'Celebrity Big Brother' has got off to a flying start and we're hoping it will retain its title as the number-one show on commercial TV in January. Peaking at a 35% share for young adults means it's a must-have for advertisers wanting to reach mass audiences."
The most-watched show during the crucial 9pm hour was home renovation programme 'DIY SOS: The Big Build' on BBC One, with an average audience of 4.49 million viewers, an 18.4% share.
Richard Desmond, Northern & Shell chairman, said when he purchased Channel 5 in 2010 that he wanted to bring 'Big Brother' to the channel. The first 'Celebrity Big Brother' in summer 2011 had an average consolidated audience of 5.57 million.