BACKGROUND: Diffusion-weighted magnetic resonance imaging (DWI) shows cerebral infarction within minutes of its occurrence, but its value in clinical management after the stroke is less clear. We evaluated DWI scans in patients with minor strokes to determine whether DWI was helpful in identifying the stroke lesion and how long after the stroke could DWI still identify the lesion. METHOD: Patients admitted to our hospital with symptoms of a lacunar or minor cortical or posterior fossa stroke underwent T2 and proton density magnetic resonance imaging (MRI) of the brain, followed by DWI on a 1.5 Tesla Siemens scanner. The individual MR sequence images were examined (blind to each other and clinical information) to identify any recent infarction. RESULTS: In 40 subjects (13 lacunes, 17 cortical, 5 posterior circulation infarctions, 2 transient ischemic attacks [TIAs] and 3 non-stroke), DWI scans showed the recent infarction clearly (even tiny ones) in 24 subjects (60%), in 12 of whom no infarction was visible on the T2 or proton density images. DWI also correctly excluded infarction in patients subsequently found not to have had a stroke. The diffusion abnormality was visible for up to 23 days after the stroke. CONCLUSION: DWI is useful for pinpointing the site of small infarctions that are either not visible or not distinguishable from previous lesions on T2 or proton density MRI, up to at least 3 weeks after the stroke. This may assist with planning further management of the stroke. The clinical use of DWI should not be restricted to just the first few hours after the stroke.