Objective:The objectives of this study were to test the value of adding serial measurements to the Survival Prediction Index (SPI 2), and to investigate whether time trajectories add predictive information beyond measurements at a single point in time. Design: Prospective clinical trial. Setting: Intensive care unit at a Veterinary Teaching Hospital. Animals: 63 critically ill dogs Interventions: Physiologic data were collected within 24 hours of admission to the ICU (Day 1), and again on Day 3 of hospitalization. Measurements: The first analysis applied the SPI 2 equation on Day 1 and again on Day 3. Then a prediction model was re-estimated using Day 1 measurements, and the incremental predictive value of adding Day 1 to Day 3 change scores was evaluated. The third analysis tested the incremental predictive value of change scores in models containing only one prognostic variable. The final analysis compared the re-estimated Day 1 model to an analogously reestimated Day 3 model. Main Results: Using the SPI 2 equation, the AUC was 7.7% higher using Day 3 measurements than that obtained using Day 1 measurements (P = 0.515). Starting with the re-estimated Day 1 model (AUC = 0.925), forward stepwise addition of the difference score for each variable did not result in an improvement in the AUC. The AUC for the reestimated Day 1 model was not statistically different from that of the re-estimated model using Day 3 measurements. Conclusions: This study shows no benefit to repeated calculation of the SPI 2 later in hospitalization. (J Vet Emerg Crit Care 2001; 11(3): 183-189)