Chromosomal translocations involving the heavy chain immunoglobulin locus on chromosome 14 and a region on chromosome 18 encoding the bcl-2 gene [t(14;18)] are a characteristic and prevalent chromosomal abnormality in nodal malignant lymphoma, particularly follicular lymphoma. Using the polymerase chain reaction on routinely processed tissue, t(14;18) has been demonstrated in 22% of primary intestinal lymphomas, i.e. in two of nine cases of malignant lymphomatous polyposis, in four of 19 cases of polymorphic B-cell lymphoma and in one of four high-grade unclassified tumours. The findings in this study contradict those of other studies which have shown no such translocation in primary gastric and small intestinal lymphoma. The presence of t(14;18) indicates heterogeneity of molecular abnormalities within histopathologically homogeneous tumours and suggests that caution should be employed in using molecular cytogenetic data to support theories of tumour histogenesis. The low prevalence of this translocation in intestinal lymphoma makes the use of such a methodology as a primary diagnostic aid doubtful, although the technique may help to distinguish primary and secondary lymphoma and could also be used to demonstrate secondary spread.