Castleman disease (CD) is a rare lymphoproliferative disorder that occurs in adults and rarely in the pediatric population. The disease is characterized by slowly enlarging masses that can form anywhere within the lymphatic system. It is an uncommon cause of a neck mass in both children and adults that presents insidiously and nonspecifically. A 21-year-old woman was referred to the otolaryngology service because of an asymptomatic neck mass found incidentally on computed tomographic imaging 15 months earlier. On repeat imaging, the lesion was characterized as a homogenously enhancing soft tissue mass and appeared stable in size compared with previous studies. Given the nondiagnostic radiologic features, tissue sampling was pursued, first using fine-needle aspiration and ultimately excisional biopsy. The excision revealed histopathology consistent with unicentric, hyaline-vascular CD. Excision is the gold standard for treatment of this variant of CD. The patient was referred to the hematology/oncology service but was subsequently lost to follow-up. This case illustrates a rare cause of a neck mass in a young adult and exemplifies the extremely broad differential in this setting. In addition, it highlights the importance of a systematic and thorough approach to diagnosing neck masses in children and adults.