Brain single photon emission CT (SPECT) scans indirectly show functional activity via measurement of regional cerebral blood flow. In conventional SPECT scans, the typical tomographic slices are produced. In three-dimensional thresholded SPECT scans, pixels representing activity below a certain threshold are discarded. A retrospective analysis of 427 patients shows that three-dimensional thresholded SPECT scans yield a sensitivity for predicting clinical attention deficit hyperactivity disorder of 54% [95% confidence interval (CI), 46%-61%; specificity, 76%; 95% CI, 71%-81%] compared with 4% sensitivity [95% CI, 2%-8%; specificity, 97%; 95% CI, 94%-98%] for conventional SPECT scans. For 170 of the patients originating from a general psychiatry practice, conventional SPECT showed 10% sensitivity (95% CI, 4%-23%) and 98% specificity (95% CI, 93%-99%), whereas three-dimensional thresholded SPECT showed 83% sensitivity (95% CI, 68%-91%) and 77% specificity (95% CI, 69%-83%). These findings indicate that a much stronger signal is obtained when the three-dimensional thresholded SPECT scan is performed rather than the conventional SPECT scan in detecting attention deficit hyperactivity disorder and suggest similar results may be obtained for other psychiatric disorders.