Background: For future health technology assessment, an assessment of the utility value of influenza-like illnesses (ILIs) is crucial. Therefore, the objective of this study was to evaluate the impact of ILIs on utility value in a Japanese population. Methods: We conducted an online survey between March and June 2024 to evaluate the impacts of ILIs on health-related quality of life, using a Japanese version of the EuroQol 5 Dimensions 5 Levels (EQ-5D-5L) and EuroQol visual analog scale (EQ-VAS). Participants were children and adults aged <80 years who experienced ILI symptoms or required home isolation due to a respiratory infection. A follow-up survey was conducted 2–3 weeks after the first survey to assess recovery. For children, we asked their parents or guardians to answer as the child’s proxy. A generalized linear model was used to assess the impact of patient demographics on disutility. Results: In total, 264 participants answered the follow-up survey (134 adult participants and 130 parents or guardians). The mean differences in the utility value and VAS score between the first and follow-up surveys were −0.067 and −14.2, respectively. In the generalized linear model, symptom onset within 7 days in the first survey was significantly associated with disutility value (coefficient −0.049 [95% confidence interval [CI] −0.086 to −0.012], p=0.010). However, none of the patient demographics were significantly associated with disutility value. Conclusions: Utility values were lower during the symptomatic phase compared with the recovery phase. Our results are useful for disease burden assessment, health technology assessment, and cost-effectiveness analysis, which can support national decision-making on the preventive and therapeutic management of respiratory infections.