It is crucial to predict drug effectiveness in chronic disease, such as dilated cardiomyopathy (DCM), in which the left ventricular (LV) function might be improved by beta-blocker therapy. As the functional improvement effected by beta-blocker therapy takes more than 2 months, we investigated whether iodine-123 metaiodobenzylguanidine (123I-MIBG) imaging could be used to predict drug effectiveness. We studied 13 patients (11 men and two women; mean age, 43+/-13 years) with DCM and seven normal subjects (six men and one woman; mean age, 48+/-16 years). We obtained myocardial single-photon emission tomography (SPET) images 15 min and 4 h after administration of 123I-MIBG (111 MBq). Studies were performed in the patients with DCM before and 1 and 3 months after the administration of metoprolol and in the normal subjects. We calculated the regional 123I-MIBG washout rate (r-WR) in the SPET image, and the global 123I-MIBG washout rate (g-WR) and heart-mediastinum activity ratio (H/M) using the anterior planar image. We classified patients into those showing a >/=5% increase in LV ejection fraction (LVEF) at 3 months compared with LVEF values before the treatment (group I, n=7) and those showing a <5% increase in LVEF (group II, n=6). In group I, the r-WR values at pretreatment and at 1 month and 3 months of treatment, respectively, were 36%+/-19%, 29%+/-14%* and 25%+/-13%* in the anterior segment, 39%+/-17%, 33%+/- 17%** and 28%+/-17%* in the lateral segment, 36%+/- 16%, 31%+/-14%* and 22%+/-12%** in the septal segment and 40%+/-11%, 37%+/-19% and 31%+/-18%* in the inferior segment; the g-WR was 45%+/-11%, 43%+/-10% and 34%+/-9%*, respectively (* P<0.05, ** P<0.01 vs pretreatment). In group II, there were no significant changes in regional or global parameters during the 3-month period. In normal subjects, the r-WR values in each of the anterior, lateral, septal and inferior segments were significantly lower than those in groups I and II. These values were 18%+/-9%, 18%+/-15%, 20%+/-12% and 21%+/-15%, respectively. This study demonstrated that with regional assessment 123I-MIBG SPET imaging can be used to predict the functional improvement of LVEF at 1 month of beta-blocker therapy in patients with DCM.