As Mathew et al1 stated, carbon dioxide levels must be considered in the analysis of regional CBF. We measured such levels routinely, and they were considered in our data analysis. There were no group differences and no correlations with blood flow in this sample. We have seen schizophrenics with unusual carbon dioxide concentrations. Among other problems, such patients may show excessive anxiety toward the procedure. No such patients were included in our study and normally are not even seen clinically since such reactions invalidate the procedure. Still, the point is valid: several studies in addition to ours (eg, Mathew et al1) did not report such results routinely and it would be an aid to readers if this were done.