The reactive strength index (RSI) is often used to quantify drop jump (DJ) performance; however, not much is known about the biomechanical determinants of the RSI. The purpose of this study was to investigate the correlations between the RSI and several biomechanical variables calculated from DJ performed with different initial drop heights. Twelve, male NCAA DI basketball players performed DJ from drop heights of 30, 45, and 60 cm. Force plates were used to calculate DJ performance parameters (i.e., DJ height, contact time, and RSI) and DJ biomechanical variables (i.e., vertical stiffness and eccentric/concentric energetics). Regression analyses were used to assess the correlations between variables at each drop height, whereas ANOVA's were used to assess the differences of all variables across drop heights. Follow-up analyses used two neural networks to determine if DJ performance and biomechanical data could accurately classify DJ trials by drop height condition. Vertical stiffness values were significantly correlated to RSI at each height, but did not change across drop heights. Surprisingly, the RSI and other DJ parameters also did not vary across drop height, which resulted in the inability of these variables to accurately classify DJ trials. Given that vertical stiffness did not change across drop height, and was highly correlated with RSI at each height, the RSI appears to reflect biomechanical behavior related to vertical stiffness during DJ. However, the inability of the RSI to accurately classify drop height condition questions the use of RSI profiles established off of DJ from different heights.