The present study reports the growth rate in two cases of main duct pancreatic intraductal papillary-mucinous neoplasms (MD-IPMNs) demonstrating significant changes over several years' observation. The first patient was a 74-year-old woman with an incidental finding of diffuse dilatation of the main pancreatic duct (MPD). Endoscopic retrograde pancreatography (ERP) identified a 5 mm filling defect. Three years later computed tomography (CT) revealed a 20 mm mass occupying the MPD. The second patient was a 67-year-old woman who presented with back pain. Abdominal CT revealed a 5 mm mass in the dilated MPD. Five years later, CT and ERP showed a 20 mm mass occupying the markedly dilated MPD. Both patients subsequently underwent pancreatectomy. Histologically, the tumors showed an intraductal papillary growth occupying the dilated MPD and comprised of mucin-containing columnar epithelial cells. The tumor volume doubling time of these MD-IPMNs was 141 and 304 days in patient 1 and 2, respectively, with a mean of 222.5 days. The present reports demonstrate the ability of benign MD-IPMNs to grow at a significant rate, supporting the current consensus guidelines that MD-IPMNs require surgical resection.