The present invention provides a composition, such as a food and pharmaceutical agent, which comprises dihomo-.gamma.-linolenic acid, and which has the effect of preventing or treating skin diseases; a composition such as a food and pharmaceutical agent which comprises dihomo-.gamma.-linolenic acid and which has the effect of preventing or treating skin diseases; and a composition which comprises dihomo-.gamma.-linolenic acid and which has the effect of preventing or treating diseases related to increased mast cell count.