Decidual cast is a little-known entity characterized by sloughing of the endometrium in several large pieces or in one cylindrical or membranous piece retaining the shape of the uterine cavity. Accounts of the diagnosis are sporadic and have not previously appeared in the pediatric imaging literature. We describe a case of a post-menarchal adolescent girl presenting with abnormal uterine bleeding, severe dysmenorrhea, and imaging features of genital tract obstruction, the cause of which was found to be a large decidual cast during examination under anesthesia. While rare, awareness of this phenomenon should be useful to pediatric imagers as the combination of bleeding and obstructive symptoms produces a confusing picture that may lead to a protracted clinical and imaging course.