Ocular adnexal MALT lymphoma (OAML) is linked to Chlamydophila psittaci (Cp) infection. Viability and infectivity of Cp, demonstrated by growth in culture, has not been yet investigated in these patients. We conducted a single-center prospective case-control study to assess the prevalence, viability and infectivity of Cp in 20 OAML patients and 42 blood donors registered in a 6-month period. The presence of Cp in conjunctival swabs and peripheral blood mononuclear cells (PBMC) of patients and donors was assessed by TETR-PCR and in vitro cultures. From an epidemiological point of view, OAML patients often resided in rural areas, and reported a history of chronic conjunctivitis and prolonged contact with household animals (85% vs. 38% of donors; p 5 0.00001). Cp was detected in lymphoma tissue in 15 (75%) patients. Cp DNA was detected in conjunctival swabs and/or PBMC from 10 (50%) patients and in PBMC from 1 (2%) donor (p 5 0.01). Viability and infectivity of Cp, demonstrated by growth in culture, were confirmed in conjunctival swabs and/or PBMC from 5 (25%) patients, but not in donors (p 5 0.002). This prospective study demonstrates, for the first time, that Cp present in the conjunctiva and PBMC of OAML patients is capable to grow and be isolated in cell cultures. Cp infection is common in OAML patients and exceptional in blood donors. Epidemiological data of OAML patients (prolonged contact with household animals and chronic conjunctivitis) are consistent with Cp exposure risk. ' 2008 Wiley-Liss, Inc.Key words: chlamydia; zoonoses; MALT lymphoma; ocular adnexal lymphoma; epidemiology Ocular adnexal MALT lymphoma (OAML), an indolent malignancy that originates in the lachrymal gland, eyelids, orbital soft tissues or conjunctiva, is associated with a persistent infection of Chlamydia psittaci (Cp) 1 (now reclassified as Chlamydophila psittaci, 2 ) the etiologic agent of psittacosis, a human infection caused by exposure to infected birds. 3 In OAML patients, Cp determines a local and systemic infection because its DNA can be detected both in the lymphomatous lesion of the ocular adnexae and, in near 40% of cases, in the peripheral blood mononuclear cells (PBMC). 1 This putative Chlamydia-lymphoma association is also supported by objective tumor regression observed in 65% of patients with Cp-related OAML treated with the antibiotic doxycycline. 4,5 To date, the comparison of Cp infection prevalence between OAML patients and controls has been actually performed on retrospective series spanning different periods, and, importantly, methods used so far did not allow us to assess whether the presence of Cp DNA and antigens in lymphoma samples and PBMC corresponded to viable and infectious microorganisms. 1 This is a relevant issue considering that the presence of viable Cp in OAML patients, demonstrated by growth in cultures, would increase the level of evidence supporting this association, fulfilling the second Koch's postulate, which states that ''the pathogen must be isolated and grown in pure culture.'' In the sett...