Saccharomyces cerevisiae cells control their cell size at a point in late G1 called Start. Here, we describe a negative role for the Sin3/Rpd3 histone deacetylase complex in the regulation of cell size at Start. Initiation of G1/S-specific transcription of CLN1, CLN2 and PCL1 in a sin3Δ strain occurs at a reduced cell size compared with a wild-type strain. In addition, inactivation of the transcriptional regulator SIN3 partially suppressed a cln3Δ mutant, causing sin3Δcln3Δ double mutants to start the cell cycle at wild-type size. Chromatin immunoprecipitation results demonstrate that Sin3 and Rpd3 are recruited to promoters of SBF (Swi4/Swi6)-regulated genes, and reveal that binding of Sin3 to SBF-specific promoters is cell-cycle regulated. We observe that transcriptional repression of SBF-dependent genes in early G1 coincides with the recruitment of Sin3 to specific promoters, whereas binding of Sin3 is abolished from Swi4/Swi6-regulated promoters when transcription is activated at the G1 to S phase transition. We conclude that the Sin3/Rpd3 histone deacetylase complex helps to prevent premature activation of the S phase in daughter cells.