Epidermal growth factor receptor inhibitors (EGFRIs) frequently cause cutaneous adverse effects such as papulo-pustular eruptions. However, the mechanism of the reactions remains unclear. We investigated whether EGFRIs have an influence on innate immune response in patients' skin to reveal the pathological mechanism of cutaneous adverse reactions caused by EGFRIs. The levels of human β-defensins (hBDs), which serve as the first line of defense against infection by pathogenic microorganisms, in the stratum corneum samples of patients treated with EGFR monoclonal antibodies (mAbs) were measured before and after starting therapy. In contrast to the findings in patients without eruptions who showed no obvious trends in hBD production, a significant decrease in hBD1 and hBD3 production was observed in patients who developed papulo-pustular eruptions. Similar changes were observed in hBD2 production. Our results may suggest that a reduction in hBD contributes to the increased incidence of papulo-pustular eruptions.