Background <p>Asymptomatic, mild urinary tract dilatation (UTD) that does not resolve can progress in severity, which suggests the need for continued observation. However, no studies have investigated the factors that contribute to the non-resolution of mild UTD.</p> Methods <p>We conducted this prospective cohort study of children who were newly diagnosed with mild UTD during the neonatal period from 2013 to 2021. The patients were evaluated by periodic kidney ultrasound until 3&#xa0;years of age. Sonographic reference values for kidney length were determined according to estimation formulas, and sonographic kidney volume was calculated using kidney length, width, and depth.</p> Results <p>This pilot study included 33 children with mild UTD, totaling 58 kidney units. The kidney units were graded as UTD P1 in 23 and UTD P2 in 35 units. Sonographic kidney length and volume were significantly higher for kidneys with UTD P2 units that did not resolve over 3&#xa0;years than in those that resolved at 3&#xa0;months, 6&#xa0;months, and 1&#xa0;year. The time to resolution of UTD P2 units in kidneys with a length of &gt; 0.7 standard deviations at 3&#xa0;months and &gt; 1.2 standard deviations at 6&#xa0;months was significantly longer than the time to resolution in kidneys with a length of ≤ 0.7 standard deviations at 3&#xa0;months (<i>p</i> &lt; 0.01) and ≤ 1.2 standard deviations at 6&#xa0;months (<i>p</i> = 0.01).</p> Conclusions <p>Increased sonographic kidney length in UTD P2 is a prognostic factor for non-resolution of mild UTD.</p> Graphical Abstract <p></p>