Sessile serrated adenomas (SSAs) are associated with colorectal carcinomas (CRCs) that demonstrate high microsatellite instability (MSI-H). Currently, SSAs are managed clinically in a similar fashion to adenomatous polyps (APs). We studied the natural history of SSA by analyzing the outcome of previously undiagnosed SSAs and comparing it with that of hyperplastic polyps (HPs) and APs. All colorectal polyps diagnosed between 1980 and 2001 as HP were selected for study. Polyps identified as possible SSAs were reviewed by 3 pathologists, and the diagnosis was confirmed. Clinical follow-up was obtained for each SSA patient and matched with control HP and AP patients. In total, 1402 colorectal polyps diagnosed as HP were examined and 81 polyps in 55 patients (5.8%) were rediagnosed as SSA. Of these, 40 SSA patients had no previous history of either CRC or AP with high-grade dysplasia (HGD). Of these 40 patients, 5 developed subsequent CRCs and 1 developed AP with HGD. The incidence of subsequent CRCs was significantly higher in SSA patients than in control patients with HP (12.5% vs. 1.8%) and AP (12.5% vs. 1.8%). All of the subsequent CRCs or APs with HGD developed in the proximal colon. Four of the 5 CRCs demonstrated a high microsatellite instability phenotype. We conclude that SSAs are high-risk lesions, with 15% of the SSA patients developing subsequent CRCs or APs with HGD. This incidence is higher than that of the control HP and AP patients, and would support close endoscopic follow-up in patients harboring SSAs.