On days 4 and 13 after an elective cesarean section, a 37-year-old woman developed sudden-onset severe occipital headaches. She had no history of migraines. On hospital admission (day 13), her blood pressure was 150/78 mm Hg and she had no neurological deficits. Blood cell counts, electrolyte levels, and urinalysis results were normal. The results of extensive tests for vasculitis were negative, including tests for antinuclear and anti–double-stranded DNA antibodies, rheumatoid factor, anti–neutrophil cytoplasm antibodies, antibodies to proteinase 3 and myeloperoxidase, and anti-Ro, anti-La, anti–Scl-70, and anti–Jo-1 antibodies, and serological tests for syphilis and Lyme disease. A cerebrospinal fluid examination showed 0 red blood cells per microliter, 2 white blood cells per microliter, and protein level of 0.044 g/dL. Transcranial Doppler studies showed elevated blood flow velocities in all circle of Willis arteries. A brain MRI (on day 13) showed multiple lesions, hyperintense on T2-weighted and fluid-attenuated inversion recovery (FLAIR) sequences (Figure 1A), without restricted diffusion on apparent diffusion coefficient (ADC) maps. The results of MR venography were normal. Her hospital course was notable for frequent headache exacerbations with blood pressure elevations up to 190/100 mm Hg. Twenty-four–hour urine metanephrine and vanillylmandelic acid levels were normal. On day 17, a second brain MRI showed resolution of most previously noted parenchymal lesions, and the results of MR angiography (MRA) of the head and neck vessels were normal. She had an acute headache exacerbation on day 20. A repeat brain MRI was normal; however, MRA showed segmental vasoconstriction of the bilateral distal internal carotid arteries, middle cerebral arteries (MCAs), and posterior cerebral arteries (Figure 2A). She was treated with intravenous magnesium sulfate and antihypertensive agents. Head computed tomographic (CT) angiography on day 22 showed complete resolution of proximal MCA and posterior cerebral artery vasoconstriction (Figure 2B); however, segmental narrowing was still present in some MCA branches. Her headaches resolved over 10 days, and she was discharged from the hospital.