False-negative diffusion-weighted (DW) imaging findings are often encountered during the acute stage of cerebral ischemia. The types of acute ischemia most likely to be missed by conventional DW imaging, and the utility of additional thin-section DW imaging of the infratentorium were investigated in 192 consecutive patients admitted within 24 hours of the onset of ischemic symptoms. If 6-mm section DW imaging at admission showed no obvious lesion, additional 3-mm section DW imaging of the infratentorium was performed. Six-mm section DW imaging failed to demonstrate ischemic lesion in 32 patients; 18 patients with transient ischemic attack (TIA), 13 with infratentorial infarction, and one with supratentorial infarction. Three-mm section DW imaging revealed the ischemic lesions in 12 of these 32 patients. Most patients with negative 6-mm section DW imaging findings at admission suffered from either infratentorial infarction or TIA. If 6-mm section DW imaging shows no ischemic lesion, 3-mm section DW imaging of the infratentorium is considered to be useful for detection of the lesion.