To evaluate the usefulness of colonoscopy for the detection of ileal involvement in patients with intestinal follicular lymphoma, seventeen patients with intestinal follicular lymphoma who underwent colonoscopy and biopsy sampling from the terminal ileum were enrolled. The patients were divided into 2 groups: cases with ileal involvement (n=6) and cases without ileal involvement (n=11). Patients' clinical backgrounds were compared between the two groups. Subsequently, 10 board-certified endoscopists independently evaluated the endoscopic pictures and determined whether the ileum was involved with follicular lymphoma. Infiltration of follicular lymphoma cells were identified in 6 patients (35.3%). Cases with positive ileal involvement were diagnosed with follicular lymphoma at a younger age than were cases without ileal involvement (55.4 +/- 7.4 vs. 68.1 +/- 10.3 years, p=0.011). Macroscopically, in patients with ileal involvement, there were multiple polypoid elevations smaller than 5 mm in 4 cases, single polypoid elevation smaller than 5 mm in 1 case, and single polypoid elevation larger than 5 mm in 1 case. In patients without ileal involvement, there were no lesions in the terminal ileum in 7 cases, and multiple polypoid elevations smaller than 5 mm were seen in 4 cases. The accuracy of the macroscopic evaluation by 10 board-certified endoscopists was 68.8%. Colonoscopy is particularly recommended during the initial workup of patients with follicular lymphoma diagnosed at age <= 60 years. The diagnosis of ileal involvement based on morphology alone is difficult; thus, biopsy and pathologic diagnosis are required for accurate diagnosis.