This paper introduces the concept of massive gender trauma, a clinical syndrome arising at the onerous intersection of the misgendering of transgender patients and the subjective, anguished experience of the natal body. Analysts have become increasingly aware in recent years of the complex interactions between psyche, soma, and culture. Consequently, the field is increasingly hospitable to considering the psychic risks inherent in misgendering. However, patients' body dysphoria is often left unaddressed even by analysts who seek to work within their analysands' gendered experience. Through a detailed, in-depth account of work with a five-year-old trans girl (female-identified, male-bodied), the developmental implications of the natal body's not becoming sufficiently mentalized in the course of treatment are tracked and explored. Attention to unconscious fantasy and its transformations shows the importance of helping transgender patients whose bodies are a source of suffering to be able to psychically represent their pain as a critical step in the process of a psychologically healthy transition.