To assess whether 'treatment day' is a significant predicting factor in Kawasaki disease and imposes a risk for coronary artery aneurysms (CAAs) in a per-day analysis. CAA formation can be reduced from 25% to 10% when treated with intravenous immunoglobulin (IVIG). Patient data from (n=1016) a single center were collected for an observational cohort study. After exclusions, we retrospectively analyzed 776 patients in total. A multivariate analysis was performed with treatment day as a continuous variable (n=691). Patients were categorized as no enlargement, small CAA, medium CAA, and giant CAA. Late treatment per-day was a significant predicting factor for the development of larger CAAs. ORs for medium and giant CAAs per delayed day were 1.1 (95% CI 1.1-1.2) P<.05 and 1.2 (95% CI 1.1-1.2) P<.05, respectively. We showed that every day of delay in treatment of patients with Kawasaki disease inherently carries a risk of medium and giant aneurysm formation. There was no cut-off point for treatment day that could mark a safe zone.