We examine the credibility and consequences of synergy disclosures in over 12,000 U.S. merger and acquisition (M&A) transactions announced between 2004 and 2021. Using press releases and conference call transcripts, we identify both qualitative and quantitative statements about expected synergies. We find that these disclosures are more common in larger deals, when more stock-financing is used, and when the targets are publicly listed. Analyst coverage, institutional ownership, and acquirers’ forecasting experience also help explain variation in disclosure practices. Disclosing firms earn higher announcement returns, particularly when disclosures are more detailed or include numeric estimates. However, these synergy expectations are often not fully realized. More intensive disclosures are associated with lower post-merger operating performance, and numeric estimates predict more frequent and larger goodwill impairments. Our findings suggest that while synergy disclosures are positively received by investors, they tend to be overoptimistic, raising concerns about the credibility of forward-looking statements in the M&A setting.