Background: Gamma-heavy chain disease is a rare disease, described so far in approximately 150 cases. The aim of this work was laboratory diagnostics of immunoglobulin heavy chain disease. Materials and methods: A 60-year-old patient was referred to the University Hospital in Ostrava for suspected marginal zone lymphoma from gastric biopsy. Staging examinations including bone marrow trepanobiopsy and PET/CT were added; special examinations required serum protein electrophoresis, immunofixation electrophoresis, determination of polyclonal immunoglobulins, free light chains, and immunoglobulin heavy/light chain pairs. Isoelectric focusing in agarose gel followed by affinity immunoblotting and SDS electrophoresis was added due to unclear findings. Results: 0.1 % of plasma cells were found in the bone marrow, of which 87 % were clonal (pathological) plasma cells, followed by the cyt cytotype LAMBDA + CD38 + CD138 + CD45 + CD19 + CD56- CD27 + CD81- CD117-. Monoclonal heavy chains were found in the patient‘s serum. No monoclonal immunoglobulin heavy or light chains were detected in urine. The PET/CT examination showed generalized lymphadenopathy, splenomegaly and inhomogeneous accumulation of fluorodeoxyglucose in axillary and appendicular skeleton, but without the presence of typical osteolytic lesions. Conclusion: Monoclonal heavy chains of immunoglobulins are a rare disease. In contrast to the detection of a complete paraprotein molecule, additional methods must be used to confirm them. The finding of monoclonal heavy chain gamma in the serum of the study patient is related to the presence of marginal zone lymphoma, which was proven from a gastric biopsy.