Purpose: To analyze the relationships between the evolution of training-load values and countermovement jump (CMJ) as an indicator of stress and fatigue in a high-level 800-m runner during a whole season, including indoor (ID) and outdoor season (OD). Methods: Over 42 weeks, daily training load was quantified as the result of the product of the intensity and volume, and it was termed load index (LI). CMJ was measured in every running session after warm-up and immediately after the last effort of the session. Other jump-related variables such as CMJ height loss, average weekly CMJ, initial CMJ of the next consecutive session, and initial CMJ of the following week were studied. Results: A significant negative relationship was observed between LI and weekly CMJ (ID: r = −.68, P < .001, common variance [CV] = 46%; OD: r = −.73, P < .001, CV = 53%), initial CMJ of the following week (OD: r = −.71, P < .01, CV = 50%), and CMJ height loss (ID: r = −.58, P < .01, CV = 34%; OD: r = −.52, P < .01, CV = 27%). A significant positive relationship was observed between LI and initial CMJ of the next consecutive session when LI values were <8 (OD: r = .72; P < .01, CV = 52%). However, from values ≥8, the relationship turned into a significant negative one (ID: r = −.74; P < .01, CV = 55%; OD: r = −64, P < .01, CV = 41%). Conclusions: CMJ may be a valid indicator of the degree of stress or fatigue generated by specific training sessions of a competitive athlete within a single session, a week, or even the following week. There could be an individual limit LI value from which the training volume does not allow a positive effect on high-speed actions such as a CMJ in the next consecutive session.