Our principal aim was to describe functional changes in dilated left atrium (LA) of children by using new applications of LA strain. We studied 66 patients (age range 0.2–22 years) consisting of 33 with LA enlargement. We utilized speckle-tracking imaging for assessment LA longitudinal strain (S) and longitudinal displacement (D). S–D loops were generated by plotting S and D data along Y and X axes, respectively. We also measured noninvasive LA stiffness index, \(=\frac{{E/e^{\prime}}}{{LA~peak~strain}}~\) (%⁻¹). Peak S in controls was 51.16 ± 19.45% versus 23.16 ± 13.66% in dilated LA (p < 0.0001). S–D loops in dilated LA group were significantly smaller compared to controls (2.62 ± 2.88 units vs. 5.24 ± 4.00 units, p < 0.01). Noninvasive LA stiffness index was higher in dilated LA group (0.77 ± 0.63%⁻¹ vs. 0.17 ± 0.07%⁻¹, p < 0.0001). A cut-off LA stiffness value of 0.25%⁻¹ was found to maximize sensitivity and specificity (84.0% and 84.85%, respectively). Children with enlarged LA demonstrate decreased peak S, abnormal S–D loops and increased LA stiffness, providing a newer insight into LA function. Evaluation of LA mechanics may be applied in future as a surrogate for left ventricular filling parameters.