Congenital complete heart block (CCHB) is seen in 1:15,000-1:20,000 live births, with risk of left ventricular (LV) dysfunction or dilated cardiomyopathy in 7%-23% of subjects. The purpose of this study was to investigate serial changes in LV size and systolic function in paced CCHB subjects to examine the effect of time from pacemaker on echocardiographic parameters. Single-center retrospective cohort analysis of paced CCHB subjects was performed. Echocardiographic data were collected before and after pacemaker placement. Linear mixed effect regression of left ventricular end-diastolic dimension (LVEDD) z-score, left ventricular shortening fraction (LVSF), and left ventricular ejection fraction (LVEF) was performed, with slopes compared before and after pacemaker placement. Of 114 CCHB subjects, 52 had echocardiographic data before and after pacemaker placement. Median age at CCHB diagnosis was 0.6 interquartile range 0.0-3.5 years; age at pacemaker placement 3.4 0.5-9.0 years; and pacing duration 10.8 5.2-13.7 years. Estimated LVEDD z-score was 1.4 at pacemaker placement and decreased -0.08 per year (95% confidence interval CI -0.12 to -0.04; P = .002) to 0.2 (95% CI -0.3 to +0.3) 15 years postplacement. Estimated LVSF decreased -1.1% per year (95% CI -1.7% to -0.6%; P <.001) from 6 months prepacemaker placement to 34% (95% CI 32%-37%) 4 years postplacement. There was no significant change in LVSF between 4 and 15 years postplacement. Estimated LVEF did not change significantly after pacemaker placement, with estimated LVEF 59% (95% CI 55%-62%) 15 years postplacement. In 52 paced CCHB subjects, estimated LVEDD z-score decreased significantly after pacemaker placement, and estimated LVSF and LVEF remained within normal limits at 15 years postpacemaker placement.