Even though anthralin is a well-established topical therapeutic agent for psoriasis, little is known about its effects and biochemical mechanisms of signal transduction. In contrast to a previous report, we found that anthralin induced time- and concentration-dependent phosphorylation of epidermal growth factor receptor in primary human keratinocytes. Four lines of evidence show that this process is mediated by reactive oxygen species. First, we found that anthralin induces time-dependent generation of H(2)O(2). Second, there is a correlation between a time-dependent increase in anthralin-induced epidermal growth factor receptor phosphorylation and H(2)O(2) generation. Third, the structurally different antioxidants n-propyl gallate and N-acetylcysteine inhibited epidermal growth factor receptor phosphorylation induced by anthralin. Fourth, overexpression of catalase inhibited this process. The epidermal growth factor receptor-specific tyrosine kinase inhibitor PD153035 abrogated anthralin-induced epidermal growth factor receptor phosphorylation and activation of extracellular-regulated kinase 1/2. These findings establish the following sequence of events: (1) H(2)O(2) generation, (2) epidermal growth factor receptor phosphorylation, and (3) extracellular-regulated kinase activation. Our data identify anthralin-induced reactive oxygen species and, more specifically, H(2)O(2) as an important upstream mediator required for ligand-independent epidermal growth factor receptor phosphorylation and downstream signaling.