Gastric extranodal marginal zone lymphoma (EMZL) often shows prolonged localised disease, but the present study demonstrated the presence of tumour cells in peripheral blood (PB) of low stage patients. We studied the presence of tumour cells in PB in gastric lymphoma patients harbouring or lacking t(11;18)(q21;q21), by real-time immunoglobulin (Ig)H allele-specific oligonucleotide-polymerase chain reaction (ASO-PCR) and API2-MALT1 PCR. Tumour cells were exclusively detected in PB of t(11;18)(q21;q21)+-EMZL patients. The presence of tumour cells in PB and gastric biopsy follow-up samples showed a good correlation in these patients, suggesting clinical relevance for monitoring of tumour cells in PB of gastric t(11;18)(q21;q21)+-EMZL patients.