Objectives Our purpose was to determine the effect of interatrial block (IAB, P-wave duration ≥120 ms) on left atrial (LA) dynamics. IAB is associated with LA enlargement (LAE). LA dysfunction is associated with decreased left ventricular filling, a propensity for LA appendage thrombus formation, and reduced atrial natriuretic peptide levels. We evaluated LA function in patients with and without IAB matched for LA size. Methods Echocardiograms with LA enlargement were analyzed. Twenty-four patients had IAB, and 16 patients without IAB formed the control group. LA volumes, A-wave acceleration times (At), LA stroke volume (LASV), ejection fraction (LAEF), and kinetic energy (LAKE) were calculated. Results The control group and patients with IAB had comparable maximal LA volume and diameter (P > .05). Patients with IAB had significantly longer At (115 ± 39 ms vs 83 ± 24 ms, P < .01) and smaller LASV (7 ± 5 mL vs 17 ± 6 mL, P < .01), LAEF (9% ± 6% vs 25% ± 8%, P < .01), and LAKE (20 ± 14 vs 65 ± 44 Kdyne/cm/s, P < .01). LAKE varied inversely with P-wave duration (r = –0.51, P < .01). P-wave duration and minimal LA volume were independent determinants of LAEF. Conclusions Patients with IAB have a sluggish, poorly contractile LA, and the extent of dysfunction is related to the degree of electrical delay from IAB. IAB should be considered a marker of an electromechanically dysfunctional LA and hence a risk factor for atrial fibrillation and congestive heart failure. (Am Heart J 2001;142:823-7.)