We characterized the association between tumor size kinetics and survival in advanced urothelial cancer patients treated with atezolizumab (anti-programmed death-ligand 1 (PD-L1), Tecentriq) using a joint model. The model, developed on data from 309 patients of a phase 2 clinical trial, identified the time-to-tumor growth and the instantaneous changes in tumor size as the best on-treatment predictors of survival. On the validation dataset containing data from 457 patients from a phase 3 study, the model predicted individual survival probability using 3 or 6-months tumor size follow-up data with an area under the receptor-occupancy curve between 0.75 and 0.84 as compared to values comprised between 0.62 and 0.75 when the model included only information available at treatment initiation. Including tumor size kinetics in a relevant statistical framework improve the prediction of survival probability during immunotherapy treatment, and may be useful to identify most-at-risk patients in "real-time". This article is protected by copyright. All rights reserved.