Background We aimed to validate two similar existing prognostic scores for early risk of stroke after transient ischaemic attack (TIA) and to derive and validate a unifi ed score optimised for prediction of 2-day stroke risk to inform emergency management. Methods The California and ABCD scores were validated in four independent groups of patients (n=2893) diagnosed with TIA in emergency departments and clinics in defi ned populations in the USA and UK. Prognostic value was quantifi ed with c statistics. The two groups used to derive the original scores (n=1916) were used to derive a new unifi ed score based on logistic regression. Findings The two existing scores predicted the risk of stroke similarly in each of the four validation cohorts, for stroke risks at 2 days, 7 days, and 90 days (c statistics 0โข60-0โข81). In both derivation groups, c statistics were improved for a unifi ed score based on fi ve factors (age โฅ60 years [1 point]; blood pressure โฅ140/90 mm Hg [1]; clinical features: unilateral weakness [2], speech impairment without weakness [1]; duration โฅ60 min [2] or 10-59 min [1]; and diabetes [1]). This score, ABCD 2 , validated well (c statistics 0โข62-0โข83); overall, 1012 (21%) of patients were classifi ed as high risk (score 6-7, 8โข1% 2-day risk), 2169 (45%) as moderate risk (score 4-5, 4โข1%), and 1628 (34%) as low risk (score 0-3, 1โข0%).Implications Existing prognostic scores for stroke risk after TIA validate well on multiple independent cohorts, but the unifi ed ABCD 2 score is likely to be most predictive. Patients at high risk need immediate evaluation to optimise stroke prevention.