BACKGROUND: Patients with unruptured intracranial aneurysms (UIAs) treated by coil embolization may experience illness uncertainty despite successful treatment. This study aimed to investigate illness uncertainty in patients with UIAs treated by coil embolization and to identify the factors affecting illness uncertainty. METHODS: This cross-sectional study involved data collected from 267 patients with UIAs who received coil embolization within 2 years at a tertiary hospital in South Korea. The data were collected through structured questionnaires and the patients' medical records. The questionnaires included information on sociodemographic characteristics, the Hospital Anxiety and Depression Scale, the Multidimensional Scale of Perceived Social Support, and Mishel Uncertainty in Illness Scale data. We collected clinical characteristics through the medical records. Data analysis was performed using descriptive statistics, an independent t test, a 1-way analysis of variance, and a Pearson correlation test. We used a multiple linear regression analysis with significant variables in the univariate analysis to determine which variables had a significant impact on illness uncertainty. RESULTS: The mean Mishel Uncertainty in Illness Scale score was 52.33 (12.93). We identified age (P = .002), education level (high school graduation, P = .014; college or higher, P < .001), anxiety (P < .001), and social support (P < .001) as factors affecting illness uncertainty. CONCLUSION: Patients with UIAs who underwent coil embolization did not exhibit severe illness uncertainty. The factors affecting illness uncertainty were age, educational level, anxiety, and social support. This study provides insight into illness uncertainty in this patient population and may help develop appropriate nursing interventions.