Summary 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 unified 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 defined populations in the USA and UK. Prognostic value was quantified with c statistics. The two groups used to derive the original scores (n=1916) were used to derive a new unified 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 0·60–0·81). In both derivation groups, c statistics were improved for a unified score based on five factors (age ≥60 years 1 point; blood pressure ≥140/90 mm Hg 1; clinical features: unilateral weakness 2, speech impairment without weakness 1; duration ≥60 min 2 or 10–59 min 1; and diabetes 1). This score, ABCD2 , validated well (c statistics 0·62–0·83); overall, 1012 (21%) of patients were classified as high risk (score 6–7, 8·1% 2-day risk), 2169 (45%) as moderate risk (score 4–5, 4·1%), and 1628 (34%) as low risk (score 0–3, 1·0%). Implications Existing prognostic scores for stroke risk after TIA validate well on multiple independent cohorts, but the unified ABCD2 score is likely to be most predictive. Patients at high risk need immediate evaluation to optimise stroke prevention.