Mucinous cystic neoplasm (MCN) of pancreas is one of the precursor lesions of pancreatic ductal adenocarcinoma. The 5-year disease-specific survival for noninvasive MCNs was 100% and 20% to 60% for those with pancreatic ductal adenocarcinoma arising in a MCN. However, the significance of T1a (≤0.5 cm) and T1b (>0.5 and <1.0 cm) carcinoma arising in MCN as defined by the upcoming American Joint Committee on Cancer, eighth edition is unclear. In this study, we examined 3 cases of MCN with T1a or T1b carcinoma and compared their clinicopathologic characteristics and survival to 46 cases of MCN with low-grade dysplasia (MCN-LGD), 7 cases of MCN with high-grade dysplasia (MCN-HGD), and 7 cases of MCN with advanced invasive carcinoma (T2 or higher T stage). The tumors from all 3 cases were submitted in their entirety in 123, 296, and 200 blocks, respectively. All 3 patients were alive with no recurrence during the follow-up of 20.0, 113.8, and 137.2 months, respectively. Similarly, none of the patients who had MCN with either LGD or HGD had recurrence or died of disease. In contrast, 5 of 7 patients who had MCN with advanced invasive carcinoma had recurrence and later died of disease with a median survival of 22.9 months (P<0.001). Our study showed that MCN with T1a and T1b carcinoma had an excellent prognosis similar to MCNs with LGD or HGD after complete tumor sampling for histologic examination. Our results along with the previous studies suggest that close follow-up, rather than aggressive systemic therapy, may be a better approach for these patients.