Mycosis fungoides (MF) is the most common primary cutaneous T-cell lymphoma which may be difficult to diagnose at the early stage. The aim of the study was to evaluate the usefulness of immunohistochemical examinations and T-cell receptor γ gene (TCRγ) rearrangement analysis in the diagnosis of MF. Immunohistochemical reactions were performed with antibodies to CD2, CD3, CD4, CD5, CD7, CD8 and CD20 cell markers of skin infiltrates. TCRγ rearrangements were detected using the method of polymerase chain reaction with the subsequent separation of products by temperature gradient gel electrophoresis. The studied group consisted of 49 patients with MF and 25 patients with inflammatory dermatoses. In the majority of MF skin infiltrates cells expressed the immunophenotype: CD2+, CD3+, CD4+, CD5+, CD7-, CD8- and CD20- did not vary significantly from phenotypes observed in chronic inflammatory dermatoses. Dominant clones with TCRγ rearrangement were detected in 85% of MF skin biopsies and in 63% of MF peripheral blood cells whereas in the control group in 11% and 14% of cases, respectively. Statistically significant differences were found in the occurrence of clonal T-cells in skin infiltrates between patients with MF and the control group. A statistical analysis of TCRγ rearrangement in peripheral blood cells did not reveal any differences only in patients with the MF in the early stage (IA) when compared with inflammatory dermatoses. Detection of TCRγ rearrangement is a valid supplement to histopathologic and immunohistochemical examination in cases of suspected MF however the diagnosis should always be based on the analysis of examinations and clinical status of patients.