99mTc HMPAO single-photon emission computed tomography (SPECT) brain perfusion was conducted in 22 mentally retarded autistic individuals and 8 patients with mental retardation as control group. Autistic patients had significantly lower perfusion than control group in the following brain regions: temporal lobes, basal ganglia, and thalami. Signs and symptoms were correlated to decreased regional perfusion in the thalamic region, but no correlation between symptoms and decreased regional perfusion in the rest of the brain lobes. Statistically, no significant differences between autistic and control group for, age, assessment of mental age, weight and head circumference, while there were statistically significant differences between autistic patients and control group related to childhood autism rating scale, and aberrant behavior checklist (irritability, lethargy, stereotypic behaviors, hyperactivity and excessive speech). The study demonstrated the effectiveness of appropriate treatment in some autistic patients with brain hypoperfusion on 99mTc HMPAO-SPECT without detection of structural changes on CT or MRI.