PURPOSE: Primary orbital non-Hodgkin's lymphoma is a mucosa-associated lymphoid tissue (MALT)-type extranodal marginal zone lymphoma. Chronic antigen stimulation is implicated as a causative agent in the development of some mature B-cell proliferations; for example, there are associations involving Helicobacter pylori with gastric or conjunctival MALT lymphoma and Chlamydia psittaci with ocular adnexal lymphoma. We examined the molecular signatures of H pylori and Chlamydia in eight orbital lymphomas. METHODS: Polymerase chain reaction (PCR) was performed on DNA extracted from microdissected lymphoma cells. H pylori was detected with the urease B and vac/m2 primers. A multiplex touchdown enzyme time-release PCR assay designed to simultaneously detect Chlamydia trachomatis, Chlamydia pneumoniae, and C psittaci was performed. Authenticity of the PCR-amplified products was verified by Southern blot hybridization. RESULTS: H pylori DNA was detected in an orbital lymphoma of a French patient who had positive serum H pylori titer. C pneumoniae, but neither C psittaci nor C trachomatis, DNA was detected in another orbital lymphoma of a Chinese patient from Hong Kong. H pylori, C pneumoniae, and C psittaci genes were not found in the other six orbital lymphomas. CONCLUSION: H pylori or C pneumoniae genomic fingerprints were detected in two of seven primary orbital MALT lymphomas. These findings provide evidence for a possible involvement of particular infectious microorganisms such as H pylori and Chlamydia in primary orbital lymphoma. These different microorganisms may play similar roles in the etiology of orbital MALT lymphomas from different geographic regions. Antibiotic therapy could be considered for orbital MALT lymphomas associated with positive infection.