Objectives <p>EQ-5D has separate three-level versions for children/adolescents (EQ-5D-Y-3L) and adults (EQ-5D-3L), assessing the same five dimensions of health-related quality of life (HRQoL) using age-appropriate language. Little is known about how differences in wording affect self-reported HRQoL assessments. This study aimed to compare the measurement properties of the EQ-5D-Y-3L and EQ-5D-3L in an adult general population sample.</p> Methods <p>A cross-sectional study was conducted in a Hungarian adult general population sample representative by age and gender (<i>n</i> = 1,196). Measurement properties, including ceiling, floor, informativity, agreement (Kendall’s tau) and known-groups validity (effect sizes) based on self-perceived health and chronic conditions were compared across instruments.</p> Results <p>EQ-5D-Y-3L and EQ-5D-3L yielded 85 and 47 unique health states, respectively. Identical health profiles were reported by 59.0%. Overall ceiling was lower using the EQ-5D-Y-3L (34.8%) than the EQ-5D-3L (46.8%), with the largest dimension-level difference for EQ-5D-Y-3L worried/sad/unhappy (56.8%) vs. EQ-5D-3L anxiety/depression (71.6%). Relative informativity was higher for all EQ-5D-Y-3L dimensions (0.20–0.75) than EQ-5D-3L (0.18–0.66). Agreement was the weakest for worried/sad/unhappy vs. anxiety/depression (0.636) and the strongest for mobility (0.841). Both instruments showed medium to large effect sizes across known-groups based on level sum scores (EQ-5D-Y-3L: 0.820–2.454; EQ-5D-3L: 0.820–2.696) and index values (EQ-5D-Y-3L: 0.754–2.362; EQ-5D-3L: 0.747–2.365), with EQ-5D-Y-3L showing higher discriminatory power in 62–69% of known groups.</p> Conclusion <p>Notable differences emerged between EQ-5D-Y-3L and EQ-5D-3L in an adult general population sample, especially in the mental health dimension, suggesting that transitions between these instruments should be treated cautiously. The EQ-5D-Y-3L may offer advantages in detecting variations in mental health, even in adult populations.</p>