Objectives: To determine the proportion of haemorrhagic strokes misdiagnosed as infarcts on computed tomography (CT) in patients with mild stroke, and the implications for health care. Methods: Patients with mild stroke presenting as inpatients or outpatients four or more days after stroke to our stroke service (catchment population 500 000) were recruited prospectively. They underwent detailed clinical examination and brain imaging with CT and magnetic resonance imaging (MRI) on the day of presentation. CT and MR images were examined independently to identify infarct, primary intracerebral haemorrhage, haemorrhagic transformation, or non-vascular lesion. Results: In 228 patients with mild stroke (median time from stroke to scan 20 days), primary intracerebral haemorrhage was identified by CT in two patients (0.9%; 95% confidence interval (CI), 0.1% to 3.1%) and MRI in eight (3.5%; 1.5% to 6.8%). Haemorrhagic transformation was identified by CT in three patients (1.3%; 0.1% to 5.6%) and MRI in 15 (6.6%; 3.7% to 10.6%). The earliest time primary intracerebral haemorrhage was not identified on CT was 11 days. Conclusions: CT failed to identify 75% of primary intracerebral haemorrhages, equivalent to 24 patients per 1000 (95% CI, 14 to 37) with mild strokes. To detect haemorrhages reliably, CT would need to have been performed within about eight days. Rapid access to neurovascular clinics with same day CT brain imaging is required to avoid inappropriate secondary prevention. Increased public awareness of the need to seek urgent medical attention after stroke should be encouraged. MRI should be considered in late presenting patients.