"Jonathan" is a 16-year-old young man who presented with severe obsessive-compulsive symptoms, including contamination fears, cleaning rituals, scrupulosity, and checking compulsions, as well as significant generalized anxiety and depressive symptoms. Approximately 3.5 years before presenting to our clinic, Jonathan was diagnosed with a pineal germinoma. Before this diagnosis, he reported no obsessive-compulsive or anxiety symptoms of note and no relevant developmental issues. Jonathan completed radiation therapy and chemotherapy per protocol ACNS0232. Treatment of the germinoma was successful; a multiplanar multisequential brain MRI with and without contrast 2 years after treatment showed a stable postoperative appearance without evidence of tumor recurrence, and no other substantial changes in brain structure were observed. Immediately after completing the germinoma treatment protocol, Jonathan exhibited a dramatic onset of obsessive-compulsive symptoms in the absence of any immune triggers. At the time he presented to our clinic, he was taking fluoxetine (40 mg/day), duloxetine (90 mg/day), and risperidone (1 mg/day), which were collectively associated with partial response.