OBJECTIVE: The relationship between infarction size determined in the acute stage and the probability of the presence of large artery stenosis or cardiac source of emboli in patients with isolated deep infarcts was evaluated. METHODS: Maximum transverse diameters of the infarcts seen in 59 consecutive patients’ T2 weighted MRI scans were prospectively recorded. Receiver operating characteristic curves were generated to analyze the optimal size that discriminate IDI with and without underlying large artery stenosis or cardiac source of emboli (LAS/CSE). RESULTS: Twenty-six IDI were in the medial cerebral artery (MCA) territory, 24 in pons and 9 in thalamus. Optimal infarct diameter for MCA IDIs was 25 mm. One of the 10 cases with small (2-14 mm) and half of the 14 cases (50%) with large (17-25 mm) pontine infarcts had LAS/CSE, but only 2 patients, one in each group had severe (≥70%) basilar artery stenosis. None had major CSE. None of the 9 cases with thalamic infarcts (8-20 mm) had LAS/CSE. CONCLUSION: : IDIs in different anatomical locations should be assessed separately. Middle cerebral artery territory IDIs with a maximum diameter of less than 25 mm are rarely associated with LAS/CSE and possibly develop due to occlusion of single lenticulostriate artery. Large unilateral pontine infarcts are usually not associated with severe basilar artery stenosis or major CSE and are probably caused by basilar artery atheromatous branch occlusion