Children with forehead port-wine stains (PWSs) are at risk of Sturge-Weber syndrome (SWS). However, most will not develop neurologic manifestations.To identify children at greatest risk of SWS.In this retrospective cohort study of children with a forehead PWS, PWSs were classified as "large segmental" (half or more of a contiguous area of the hemiforehead or median pattern) or "trace/small segmental" (less than half of the hemiforehead). The outcome measure was a diagnosis of SWS.Ninety-six children had a forehead PWS. Fifty-one had a large segmental PWS, and 45 had a trace/small segmental PWS. All 21 children with SWS had large segmental forehead PWSs. Large segmental forehead PWSs had a higher specificity (0.71 vs 0.27, P .0001) and a higher positive predictive value (0.41 vs 0.22, P .0001) for SWS than any forehead involvement by a PWS.Retrospective study at a referral center.Children with large segmental forehead PWSs are at highest risk of SWS.