The behavior of pancreatic mucinous cystic neoplasms has long been debated. Some authors contend that histologically benign neoplasms can recur and metastasize. We reviewed the gross and microscopic findings and outcomes of 61 mucinous cystic neoplasms diagnosed at The Johns Hopkins Hospital from March 20, 1984 to July 8, 1998. Each neoplasm was placed into one of four categories based on complete histologic examination: invasive mucinous cystadenocarcinoma, mucinous cystic neoplasm with in situ carcinoma, borderline mucinous cystic neoplasm, and mucinous cystadenoma. Neoplasms in the latter three categories were included only if they were entirely resected and completely examined. Patient outcomes were obtained from hospital records and patient and physician follow-up. Twenty (33%) of the patients had invasive mucinous cystadenocarcinomas, and they had 2- and 5-year disease-specific survival rates of 67% and 33% (mean follow-up of survivors, 4.2 years), respectively. Nine (15%) patients had mucinous cystic neoplasms with in situ carcinoma (mean follow-up of survivors, 4.1 years). Five (8.2%) patients had borderline mucinous cystic neoplasms (mean follow-up of survivors, 5.6 years). Twenty-seven (44%) patients had mucinous cystadenomas (mean follow-up of survivors, 5.1 years). No mucinous cystadenoma, borderline mucinous cystic neoplasm, or mucinous cystic neoplasm with in situ carcinoma recurred or metastasized. No patient with the diagnosis of mucinous cystadenoma, borderline mucinous cystic neoplasm, or mucinous cystic neoplasm with in situ carcinoma died of disease. The difference in disease-specific survival rates between patients with invasive mucinous cystadenocarcinomas and those with noninvasive tumors was significant (p < 0.0001, log-rank test). One case, originally showing only benign histology on incisional biopsy, contained foci of invasive carcinoma on complete resection. Completely resected and entirely examined mucinous cystadenomas, borderline mucinous cystic neoplasms, and mucinous cystic neoplasms with in situ carcinoma follow benign courses. Because invasive carcinoma can be focal, failure to study an entire mucinous cystic neoplasm may result in the miscategorization of a malignant neoplasm as benign.