A previously healthy 10-year-old boy presented with progressive encephalopathy with regression of cognitive skills, deterioration in school performance, behavioral and personality changes, marked bradypsychia, and weight loss. Six months after clinical onset, magnetic resonance imaging results showed a subacute crescent-shaped subdural effusion in the left hemisphere with inward displacement of subarachnoid veins and effacement of convexity sulci (Figure 1). A repeat magnetic resonance imaging study 1 month later revealed an enlargement of the hematoma due to new bleeding with a fluid-fluid level and a midline shift. Subarachnoid spaces and the cerebral ventricular system were enlarged (Figure 2). The patient had not experienced any significant trauma and his coagulation parameters were normal. The neuropsychiatric symptoms remained despite a surgical evacuation of the subdural hematoma. On further investigation, laboratory test results showed increased levels of plasma homocysteine (19.5 mg/L [to convert to micromoles per liter, multiply by 7.397]; normal range, 0.7-2 mg/L) and methylmalonic acid (67.8 μmol/L; normal range, 0.08-0.58 μmol/L). Genetic testing results confirmed the diagnosis of methylmalonic aciduria and homocystinuria due to cobalamin D disease, a rare autosomal recessive disorder of cobalamin metabolism caused by mutations in the MMADHC gene1 that can result in isolated homocystinuria, isolated methylmalonic aciduria, or combined methylmalonic aciduria and homocystinuria.2 Only a few patients with cobalamin D disease have been described, and those with the methylmalonic aciduria and homocystinuria phenotype typically present with developmental delay, seizures, hypotonia, lethargy, and behavioral disorders.3 Brain abnormalities are common in inborn errors of cobalamin metabolism and include white matter changes, basal ganglia lesions, hydrocephalus, and diffuse cerebral atrophy with an enlargement of extra-axial spaces and, consequently, an increased risk of subdural hematoma with only minor trauma.4