The Tei index is an indicator of systolic and diastolic myocardial performance. We evaluated the Tei index in patients undergoing cardiac resynchronization therapy (CRT).Forty-two patients were studied before CRT and 1 day and 6 months after CRT, comparing responders with nonresponders.The Tei index decreased 1 day after CRT (left ventricle [LV]: P < .001, right ventricle [RV]: P = .01) and remained lower at follow-up (LV and RV: P < .001 vs baseline). Responders had a higher LV Tei index at baseline (P = .003) and achieved a sustained improvement in Tei index at follow-up (LV: P < .001, RV: P = .002) in contrast with nonresponders (LV and RV: not significant). Baseline LV Tei index and change in LV Tei index were both correlated with LV end-systolic volume reduction after CRT (r = 0.52, P < .001, r = 0.43, P = .006).The baseline LV Tei index was significantly higher in responders and exhibited an acute and sustained improvement after CRT. The baseline RV Tei index was similar in responders and nonresponders but improved significantly only in responders.