Methylphenidate is a frequently used treatment choice in attention deficit hyperactivity disorder. In this paper we will present a prepubertal boy who suddenly developed bilateral gynecomastia after sustained release methylphenidate (OROS MPH) treatment and we will discuss literature on this topic. Although the mechanism is not known, rare development of gynecomastia should be kept in mind in cases using methylphenidate. [Anadolu Psikiyatri Derg 2016; 17(0.300): 74-76]