Alterations of regional cerebral blood flow (rCBF) in prefrontal cortex and the anterior cingulate cortex are conspicuous imaging findings in patients with major depression (MD). While these rCBF changes have been suggested as functional disease markers, data in large patient samples examining treatment response prediction to antidepressant therapy are limited. This study examined the predictive value of Tc-99m-HMPAO-SPECT for subsequent treatment response to antidepressant therapy with citalopram in an unprecedented large collective of patients. Ninety-three patients with MD were examined with Tc-99m-HMPAO-SPECT twice, at the beginning of citalopram-treatment (T1) and after 4 weeks of treatment (T2). To determine the impact of rCBF changes associated with treatment response, the patient sample was divided into two subgroups: responders (44 patients) and non-responders (49 patients). A two-sample t-test was used to determine group-specific rCBF-differences. Age, gender and initial Hamilton Rating Scale for Depression (HRSD) were treated as regressors of no interest. The responder group revealed significant relative rCBF increases at T1 in a large region en-compassing predominantly prefrontal and temporal cortices as well as subgenual cingulate cortex. No relative rCBF decreases were detected in this group. The comparison between T1 and T2 revealed trends of rCBF decreases in inferior frontal gyrus and rCBF increases in premotor cortex in the responder group. Our data show that rCBF measurements with TC-99M-HMPAO-SPECT provide a predictor estimate for subsequent treatment response in depressed patients undergoing antidepressant therapy with citalopram. This effect is highly significant and, most notably, independent of the initial HRSD score.