UNLABELLED: To evaluate whether pre-treatment coronary artery aneurysm (CAA) Z-score, inflammatory markers, and incomplete Kawasaki disease (KD) status are associated with intravenous immunoglobulin (IVIG) retreatment and early coronary outcomes in infants aged < 6 months. We retrospectively reviewed infants aged < 6 months who were treated with IVIG for KD between 2011 and 2024. Clinical, laboratory, treatment, and serial echocardiographic data were collected, and patient-level maximal coronary Z-scores were calculated. The primary outcome was additional IVIG; the secondary outcome was CAA Z-score (≥ 2.5) at approximately 1 month after illness onset. Among 76 infants (median age, 4.3 months), 17/76 (22.4%) required additional IVIG, 14/76 (18.4%) had CAA at approximately 1 month after illness onset, and 3/76 (3.9%) had residual CAA at or after 6 months. Additional IVIG was independently associated with pre-treatment CAA (Z ≥ 2.5), earlier illness day at initial IVIG, and higher neutrophil-to-lymphocyte ratio; the multivariable model showed good discrimination (area under the curve [AUC] 0.84). For CAA at 1 month, both a baseline-only model including pre-treatment CAA Z-score, albumin, and illness day at initial IVIG (AUC 0.91, 95% confidence interval [CI] 0.84-0.97) and a course-aware model including pre-treatment CAA Z-score and additional IVIG (AUC 0.90, 95% CI 0.84-0.97) showed high discrimination. CAA at 1 month with Z-max ≥ 5.0 was associated with residual CAA risk. CONCLUSIONS:  In infants aged < 6 months with KD, pre-treatment coronary involvement and higher inflammatory burden were strongly associated with additional IVIG, whereas pre-treatment coronary disease severity, lower albumin, and an early refractory course were strongly associated with CAA at 1 month. WHAT IS KNOWN: • Infants aged < 6 months with Kawasaki disease (KD) have higher non-response to intravenous immunoglobulin (IVIG) and more coronary artery abnormalities (CAA). • Guidelines recommend early transthoracic echocardiography and coronary Z-score assessment, but infant-specific prognostic data remain limited. WHAT IS NEW: • In this infant-only cohort, pre-treatment CAA Z-score ≥ 2.5 and neutrophil-to-lymphocyte ratio ≥ 2.15 were associated with additional IVIG. • Pre-treatment CAA Z-score, lower albumin, and an early refractory course were the factors most strongly associated with CAA at 1 month.