Influence of image quality on lesion detectability in I-123 IMP brain SPECT was discussed in forty-two cases with various cerebrovascular disorders. Fifty nine lesions, which were considered to show decreased uptake of I-123 IMP, were analyzed in this study. In all cases, SPECT examinations were started within 30 minutes after intravenous administration of 3 mCi of I-123 IMP. To determine the lesion detectability, fourteen radiologists from nine hospitals were gathered and film reading was carried out. To classify the image quality into valuable or not, findings of five points on SPECT image were visually evaluated; 1) uniformity of cortex accumulation, 2) separation of bilateral visual cortices, 3) density of white matter, 4) separation between basal ganglion and thalamus, 5) density of basal ganglion and thalamus. Mean lesion detections were 10.3/14 for the lesions of cerebral cortex, 9 for cerebellum, 3.58 for white matter and 3.22 for basal ganglion and thalamus, respectively. The detectability was certainly influenced by the image quality in the lesion of basal ganglion and thalamus, white matter and single lobe of cerebral cortex. We concluded that detection of small lesions and deep lesions with I-123 IMP SPECT was dependent on the image quality and the proposed classification of I-123 IMP SPECT image was useful for evaluating image quality from the points of view of the lesion detectability.