Background: Circadian rhythms are implicated in timing of stroke onset and infarct progression in adults, but this has not been studied in pediatric/young adult populations. Methods: We queried the RAPID Insights database from centers in USA for unique patients <25 years with a CTP (10/05/2018-09/29/2023) and a minimum ischemic core volume (defined as relative cerebral blood flow (rCBF) reduction of <30%) of >0 cc and minimum mismatch of >0 cc. Imaging time was subdivided into three epochs: Nigh (23:00 h-06:59 h), Day (07:00 h-14:59 h), and Evening (15:00 h-22:59 h). We analyzed age by pre-defined strata: <2 years, 2-5, 6-11, 12-18 and 19-25. Perfusion parameters (core, perfusion volume, mismatch ratio) were analyzed using descriptive statistics. Results: 836 patients were included; 52.3% were in the 19-25 category. Median ischemic cores were larger during the Night (23.0cc [10.0 – 58.0]) compared to Day (19.0cc [8.0-42.0]) or Evening (15.0cc [7.0-33.0]), p=0.009. There was a trend towards larger perfusion volumes in the Night epoch. In the 19-25 group, perfusion volumes were significantly larger at Night (127.5cc [51.5 – 203.5]) compared to Day (74.0cc [33.0 – 139.0]) or Evening (76.0cc [38.0 – 157.5]), with larger mismatch volumes at Night. Conclusions: This is the first study to demonstrate diurnal fluctuations in perfusion parameters in a predominantly pediatric cohort.