Children with forehead port-wine stains (PWS) 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 forehead PWS, PWS were classified as "large segmental" (half or more of a contiguous area of hemi-forehead or median pattern) or "trace/small segmental" (less than half of the hemi-forehead). The outcome measure was a diagnosis of SWS.Ninety-six children had forehead PWS. Fifty-one had large segmental and 45 had trace/small segmental PWS. All 21 children with SWS had large segmental forehead PWS. Large segmental forehead PWS had higher specificity (0.71 vs 0.27, p<0.0001) and higher positive predictive value (0.41 vs 0.22, p<0.0001) for SWS than any forehead involvement by PWS.Retrospective study at a referral center.Children with large segmental forehead PWS are at highest risk of SWS.