Purpose Longitudinal analysis of health-related quality of life (HRQoL) remains unstandardized and compromises comparison of results between trials. In oncology, despite available statistical approaches, results are poorly used to change standards of care, mainly due to lack of standardization and the ability to propose clinical meaningful results. In this context, the time to deterioration (TTD) has been proposed as a modality of longitudinal HRQoL analysis for cancer patients. As for tumor response and progression, we propose to develop RECIST criteria for HRQoL. Methods Several definitions of TTD are investigated in this paper. We applied this approach in early breast cancer and metastatic pancreatic cancer with a 5-point minimal clinically important difference. In breast cancer, TTD was defined as compared to the baseline score or to the best previous score. In pancreatic cancer (arm 1: gemcitabine with FOLFIRI.3, arm 2: gemcitabine alone), the time until definitive deterioration (TUDD) was investigated with or without death as event. Results In the breast cancer study, 381 women were included. The median TTD was influenced by the choice of the reference score. In pancreatic cancer study, 98 patients were enrolled. Patients in Arm 1 presented longer TUDD than those in Arm 2 for most of HRQoL scores. Results of TUDD were slightly different according to the definition of deterioration applied.