Using Statistical Parametric Mapping (SPM), we have implemented methods to investigate the relation between findings from structural MRI and regional cerebral blood flow (rCBF) SPECT performed in groups of subjects with neuropsychiatric disorders and healthy controls. Using a MATLAB program developed and integrated into SPM, suited means of gray matter values (MGM) were calculated in regions relevant to the disorder in question, and linearly correlated with rCBF values from the same patients. In a study of Obsessive Compulsive Disorder (OCD), patients showed a focus of increased MGM in the right putamen relative to controls. MGM from the putamen were significantly inversely correlated with rCBF measures in regions critical to OCD, mainly the anterior cingulate gyrus. These results are consistent with the notion that an imbalance of cortico-striatal circuits is relevant to OCD pathophysiology. In a study of Alzheimer's disease (AD), foci of decreased gray matter in patients relative to controls were identified bilaterally in the hippocampus. In AD patients, reduced hippocampal MGM correlated significantly with decreased rCBF during a memory task, in a network of regions usually involved in memory processes. An inverse correlation with frontal rCBF was also observed, suggesting compensatory efforts of executive regions during the memory task.