Lymphomatous polyposis (LP) is a subtype of non-Hodgkin's lymphoma manifested by numerous polyps affecting long segments of the gastrointestinal tract. The malignant cells of LP often share morphological and immunophenotypic similarity with cells of nodal-based mantle cell lymphoma. Recent genetic studies have shown that mantle cell lymphomas frequently possess a characteristic translocation of the JH/bcl-1 loci. In this study, polymerase chain reaction (PCR) and Southern blot analysis were used to show the presence of JH/bcl-1 translocation in a typical case of LP of the gastrointestinal tract. This provides strong molecular evidence for a biologic link between LP and mantle cell lymphoma. The findings also imply that detection of this translocation may be useful in the diagnosis of morphologically equivocal gastrointestinal biopsy specimens.