Background: Erythroderma is an inflammatory disorder. It has various differential diagnoses, among which one of the most important is mycosis fungoides. Erythroderma itself can be a challenging disorder. Diagnosis of a mycosis fungoides patient presenting with erythroderma specially requires a careful assessment of the peripheral blood. Studies such as CD markers can lead to a more accurate diagnosis of mycosis fungoides.Methods: In this study, we evaluated ten erythrodermic patients in order to identify the source of their erythema. The underlying causes were both benign such as eczema, psoriasis, pityriasis rubra pilaris, acute generalized exanthematous pustulosis and malignant like hypereosinophilic syndrome and mycosis fungoided. Results: The CD4/CD8 ratio was greater than 10 in 2 out of 10 erythrodermic patients. These patients had decreased levels of CD7 and CD26 expression. While one of the two patients fulfilled the criteria of hypereosinophilic syndrome, the other one did not have a documented clincopathologic diagnosis and had a pathology report of lichenoid tissue reaction with eosinophilia in favor of drug reaction. Both patients had decreased levels of CD7 and CD26 expression.Conclusion: Since pathology is usually non-specific and cannot differentiate between the causes of erythroderma in erythrodermic patient with CTCL, peripheral blood findings including flow cytometry and the analysis of CD4/CD8, CD7, CD26 and CD27 expression are useful techniques which can be used for a prompt diagnosis.