Parents experience uncertainty when their children become sick. The study aimed to develop a Parents’ Uncertainty regarding their Child with Acute Illness Scale (PUCAS) and to clarify differences in PUCAS scores between groups that were divided according to participants’ demographic characteristics. PUCAS was developed based on interviews, literature review, and a pilot study. We obtained valid responses from 235 parents with children hospitalized due to an acute childhood illness. Exploratory factor analysis narrowed the number of items to 25, divided into the following 5 subscales: unpredictability of the course of the illness, ambiguity about the severity of the illness, ambiguous appropriateness of management, discrepancy of judgement with health care professionals, and lack of information about the causes of the illness. There was satisfactory construct validity and criterion-related validity. Cronbach’s alpha was 0.92 for the overall scale. Participants who used ambulances, those who were not given any reliable diagnosis for their children, and fathers scored significantly higher on the PUCAS. PUCAS has high validity and reliability in measuring uncertainty of parents who have children with acute childhood illness and could be a useful screening tool for parents with high uncertainty in a clinical setting.