The transient characteristics of high-injection-pressure diesel fuel sprays have been investigated by high-speed photography. An off-engine fuel injection system with a hydraulic unit injector was designed for spray visualization experiments. Experiments were conducted using different geometry nozzles under different operating conditions to investigate the effects of injection pressure, nozzle inlet condition, nozzle injection angle, and nozzle hole size on spray characteristics. Measurement of spray tip penetration length and spray angle were made based on the spray images. The Sauter mean diameter (SMD) averaged over the spray was measured using a light extinction method. The results of these experiments show that spray geometry (penetration length and cone angle) is sensitive to small changes in operating conditions and injector geometry. Higher injection pressures produced larger penetration, smaller spray cone angles, and smaller overall average SMD. Smaller nozzle hole sizes and smaller injection angle nozzles also produced smaller droplet sizes. Previous results from engine emissions tests using the same injector under similar operating conditions were examined in light of the observed spray characteristics. While the engine emission results are affected by many parameters related to the fuel injection system as well as many which are unrelated to the injection system, the results of these comparisons indicate that spray SMD and spatial distribution have a significant effect on engine emissions for the conditions tested.