Masks appear in several of Eugene O'Neill's early plays, serving not as a mere technical innovation, but rather as a way to explore "hidden conflicts" of human nature. This article explores three factors that could have provoked this sensitivity to the motif of masking. First is the influence of Greek tragedy--itself a domain of "the masked God"--which O'Neill tried to restore through several of his plays. Second is O'Neill's insight into the connection between modern man's selfhood and the process of masking, several decades before psychoanalysts have approached it scientifically, in complete accordance with Kohut's hypothesis about the "anticipatory function of art". The final factor consists of both his early development and his later life--especially his three marriages--as revealed through extensive biographies, correspondence, and plays. The article suggests that the most important influence on O'Neill's sensitivity to motif of masking was his mother's morphine addiction.