A 36-year-old man presented with recurrent urinary obstruction and an enlarged, partially cystic prostate tumour on ultrasonography. Microscopically, the tumour was composed of cystically dilated ducts with leaf-like stromal projections in the lumen. A portion of the stroma was very cellular with atypia and a high mitotic rate. In addition, there was a small focus with well differentiated rhabdomyoblasts. This case is the second description of a cystosarcoma phyllodes of the prostate with rhabdomyoblasts differentiation. With radical surgery and chemotherapy only partial remission could be obtained.