Peripheral T-cell lymphoma, not otherwise specified (PTCL-NOS) have an aggressive, life-threatening course. 5‑year survival rate is less than 20 %, which may be due to not timely diagnosis. PTCL-NOS can histologically and immunophenotypically mimic other T-cell lymphomas of the skin, including mycosis fungoides. In this connection, the correct diagnosis is most often established in the late stages of the disease. We present a clinical case of PTCL-NOS misdiagnosed as mycosis fungoides.