The authors have analyzed and compared the results of the 99mTc-ECD-SPECT and FDG-PET examinations, performed in alert state, of 12 children suffering from infantile (9 subjects) or atypical (3 subjects) autism. In addition to frontally increased FDG metabolism, a decreased blood flow with left-sided dominance was found bifrontally and bitemporally in the infantile form (perfusion-metabolism mismatch). The regional differences in cortical FDG uptake were not significant in atypical autism, although both the blood flow and the metabolism of the thalami were decreased. Based on the results, the authors suggest that, beyond the usually inconclusive structural (CT or MR) examinations, cerebral blood flow SPECT and metabolic FDG-PET investigations may be useful in classifying the disease.