Background/Aims: We report a case of solitary Peutz-Jeghers-type hamartomatous polyp of the duodenum in a 22-year-old Japanese woman along with the results of genomic analysis. Methods/Results: The patient was almost asymptomatic, though endoscopic examination revealed a solitary lobular polypoid lesion measuring 3 cm in diameter in the first portion of the duodenum. The lesion was resected endoscopically. Histopathological examination showed hyperplasia with a tree branch-like extension of the lamina propria derived from the muscularis mucosae, consistent with histological features of polyps of Peutz-Jeghers syndrome (PJS). No mucocutaneous pigmentation of the skin was evident and family history was negative. Analysis of the loss of heterozygosity at the locus of 19p 13.3 and mutation analysis of the STK11/LKB1 gene, which has recently been recognized as a susceptible gene in PJS, were performed. However, no evidence of genomic abnormality was found. Conclusion: The clinical and investigative findings in our case suggest that the solitary Peutz- Jeghers-type hamartomatous polyp can be regarded as a clinical entity separate from PJS.