Doctors managing acute stroke are expected to recognise signs of early infarction on CT before choosing thrombolytic treatment, according to recent trials and guidelines. The ability of 13 physicians and two neuroradiologists to recognise early infarct signs and decide whether patients should be randomised in a hypothetical stroke treatment trial was tested. Only 65% of the CT scans from 14 stroke patients were correctly identified as normal or abnormal (95% CI 60-69%). Neither observer experience nor knowledge of symptoms significantly improved recognition of abnormality, although experience did significantly improve the observers' ability to reproduce their results. Parenchymal hypodensity was the least well recognised sign. Only 45% (95% CI 40%-50%) of patients were identified correctly for the hypothetical acute stroke treatment trial. Early infarction on CT is not well recognised even by experienced doctors. Part of the problem may be in understanding the definitions of the extent of infarction. These difficulties should be considered in the design of acute stroke treatment trials and in the introduction of any new acute stroke treatments.