OBJECTIVES: In economic evaluations, health-related quality of life (HRQoL) is commonly expressed as health state utility values to derive quality adjusted life years. The objective of this study was to estimate health utilities using joint model which simultaneously models patient’s longitudinal EQ-5D trajectory and survival by accounting for the association between the two outcomes. METHODS: We analysed data from a large (n=389), repeated measure (2,450 observations), EQ-5D-3L dataset from the GATSBY trial to estimate utility values. The EQ-5D was converted into utility value using the UK tariff. Two joint models using current value association structure were fitted to predict utility values according to baseline patient characteristics and key clinical outcomes. The set of variables considered for the joint models included: baseline age, ethnicity, treatment, sex, visit (time), hospitalisation due to adverse event (AE), disease progression and time leading to death. Final set of variables were selected using likelihood ratio test (p<0.05). RESULTS: The median follow up visit was 5 months (range: 0-45 months). Time was included as a random effect. Included variables demonstrated evidence of an important association with HRQoL outcomes based on magnitude and significance of effect (p<0.05). Both models showed a highly negative statistically significant association indicating that a lower value of utility increases the risk of death. Model 1 showed disease progression was associated with a statistically significant utility decrement of 0.073. Model 2 showed that in addition to disease progression, patients’ proximity to death showed statistically significant utility decrement of 0.37 in the 3 months leading to patient’s death. CONCLUSIONS:  An event based analysis using a joint model better explained variation in EQ-5D data according to key clinical outcomes and patient characteristics. Our analysis suggests that for patients with HER2+ mGC, time leading to death is a considerable determinant for HRQoL and should be accounted for within economic evaluations.