We present the case of an adolescent girl with Dandy Walker Variant and treatment-resistant bipolar disorder. A 13-year-old girl diagnosed with bipolar disorder at age 11 was brought in for increasingly worsening aggressive behavior toward persons and objects at home and in school. She had no history of drug/alcohol use and no significant medical history. She had multiple psychiatric admissions and treatment with neuroleptics and mood-stabilizers, with minimal efficacy in controlling her symptoms. In the past, she had been placed in multiple foster homes because of aggressive behavior. Maternal pregnancy was described as full-term, with no known maternal exposure to infectious diseases. Developmental milestones were normal, and there was no family history of psychiatric illness. During this admission, psychological testing was done, with the WISC-IV showing an IQ of 72; the Bender-Gestalt test suggested serious perceptual problems; the Woodcock Johnson–III result was 2 years below chronological age; the Projective Personality Test suggested an immature youngster who has problems with attention, organization, and impulse-control. Pertinent physical examination showed macrocephaly (head circumference >60 cm), with nonextinguishing glabelar sign. The patient was an obese girl with a Body Mass Index of 46.6. She was noted on mental status examination to be withdrawn, with monotonous speech and flat affect. EEG was normal, ruling out seizure disorder. A computed-tomography scan of the brain without contrast showed Dandy Walker Variant, with inferior vermian hypoplasia, communication between the cisterna magna and the fourth ventricle, and a dilated fourth ventricle, with a normal posterior fossa. MRI of the brain confirmed the findings; the Fragile-X DNA test was normal. The patient's aggressive behavior continued, despite optimized doses of mood-stabilizers with antipsychotics. Propranolol was added, with mild improvement in her aggressive behavior. She was discharged to a residential treatment facility.