This article presents a previously unreported symptom of obsessive-compulsive disorder. The young people reported describe a fear of turning into someone or something else or taking on unwanted characteristics. We have called this transformation obsession. The bizarre nature of this obsession had led to misdiagnosis and inappropriate treatments in a number of these patients. Recognition of this symptom as an ordinary obsession (unwanted, intrusive, repetitive, and associated with an anxiety-reducing compulsion or avoidance)facilitates treatment with cognitive-behavioral therapy for obsessive-compulsive disorder. Consideration is given to screening for this obsession and whether its presence indicates a subtype of obsessive-compulsive disorder. (Contains 1 table.)