Photodynamic therapy is an alternative method for cancer treatment in which a photosensitizer exposed to a light source of suitable wavelength is excited and can subsequently react through free radical mechanisms. Recently, oxygen free radical-mediated changes in gene expression have been established. The present study shows the effect of photoactivated hypericin on the expression of the human epidermal growth factor receptor 2 (HER2) oncogene at both the mRNA and the protein level in SKBR-3 and MCF-7 breast adenocarcinoma cell lines. The photodynamic therapy-induced decrease in mRNA expression was reversed by the singlet oxygen scavenger trolox, which supports a role for singlet oxygen. In addition, prevention of the generation of reactive oxygen species by pretreatment with trolox effectively blocked the antiproliferation activity of photoactivated hypericin. These results may have important implications at least for recurrent breast cancer with HER2 expression alone or in combination with conventional therapies.