Among 821 consecutive patients admitted to an acute stroke unit, the initial diagnosis of stroke proved incorrect in 108 (13%). The commonest causes of misdiagnosis were unwitnessed or unrecognised seizures (39%) and confusional states and syncope (24%), conditions that can be diagnosed only clinically. In a series of 93 cases examined post mortem, the diagnostic accuracy for stroke varied with clinical skill and ranged from 38% to 89%. The frequency (but not the type) of incorrect diagnosis was the same in 244 patients investigated with computerised tomographic (CT) scanning as in 345 patients investigated without it. Although CT and other neurological investigations are useful aids in the diagnosis of stroke, they remain a supplement to, and not a substitute for, correct clinical evaluation.