We describe the case of a 60-year-old man who, after seven years with eczematous changes, developed erythrodermia, clinically suspect of mycosis fungoides. T-cell receptor gamma (TCR-gamma) gene rearrangements were retrospectively investigated by polymerase chain reaction (PCR). DNA from a formalin-fixed, paraffin-embedded punch biopsy showed a monoclonal pattern compatible with mycosis fungoides two years before the diagnosis was confirmed histopathologically. PCR was also used to evaluate the effect of different treatments. This case demonstrates the value of PCR for early diagnosis and evaluation of treatment of cutaneous T-cell lymphoma.