The frequency of gastrointestinal (GI) tract involvement in mantle cell lymphoma (MCL) at diagnosis is reported to be below 30%. To investigate the actual frequency of GI involvement by MCL, upper and lower endoscopy was prospectively performed on 13 untreated MCL patients at diagnosis. Multiple biopsies from endoscopically normal and abnormal gastric and colonic mucosa were studied with immunohistochemistry (IHC) for CD20, CD5, and cyclin D1, as well as fluorescence in situ hybridization (FISH) for t(11;14) and polymerase chain reaction (PCR) for immunoglobulin heavy chain gene. Abnormal mucosa was identified in 38% of cases by upper endoscopy (mainly mild nonspecific gastritis) and in 54% of cases by lower endoscopy (mostly micropolyps). Histologically, infiltration by MCL was demonstrated in the stomach in 77% of cases and in the colon in 77% of cases. As a whole, 92% of patients showed upper or lower GI tract infiltration by MCL. Histologic evidence of MCL involvement was present in all cases with endoscopically abnormal mucosa, but it was also observed in two-thirds of cases with endoscopically unremarkable mucosa. Positive cyclin D1 IHC was seen in all instances displaying CD20 and CD5-positive lymphoid infiltrates, whereas t(11;14) was demonstrated by FISH in 63.5% and PCR was clonal in 64% of those instances. In conclusion, the great majority of MCL patients showed GI tract involvement at the time of diagnosis, not uncommonly in the form of minute lymphoid infiltrates. IHC for cyclin D1 was significantly more sensitive than FISH t(11;14) or PCR for immunoglobulin heavy chain gene to confirm MCL in this setting.