OBJECTIVE To determine whether the size of the birthmark in patients with Sturge-Weber syndrome (SWS) who have brain involvement can help predict neurologic disability. STUDY DESIGN Fifty-one patients with SWS with facial birthmarks and brain involvement documented on magnetic resonance imaging were included in this retrospective chart review. A neuroradiologist, blinded to all clinical information, assigned a previously validated SWS neuroimaging score. A pediatric neurologist prospectively assigned previously validated neurologic severity scores, based on seizures, hemiparesis, visual field cut, and cognitive impairments. Three raters, blinded to clinical scores, independently graded the size of facial birthmark in each patient based on photographs. Their scores were averaged. Birthmark scores were compared with the imaging and neurologic severity results using nonparametric correlation analysis. RESULTS Size of facial port-wine birthmark correlates with magnetic resonance imaging scores on the left and right sides (ρ = 0.57 and 0.66 [P < .001], respectively). Size is also positively associated with the neurologic severity rating for patients age 6 years and above (1-sided Fisher exact, P = .032). CONCLUSIONS The size of facial port-wine birthmark in SWS brain involvement can be developed as a tool to predict neurologic severity of the disease.