Sudden death syndrome (SDS), caused by Fusarium virguliforme, causes substantial yield losses in soybean. However, relationships between components of disease progress, including time of disease onset, and soybean yield are poorly understood. Individual soybean plants and 1.5 x 3 m quadrats were monitored in commercial fields and experimental plots to quantify the impact of SDS foliar symptom onset on SDS intensity, soybean yield components and yield, in Iowa. The day of year when SDS foliar symptoms were first detected (onset time) and progress of SDS incidence and severity were recorded weekly. Individual soybean plants and quadrats were harvested at the end of each season. Linear regression showed that SDS onset time had a strong negative relationship with SDS intensity and a strong positive relationship with soybean yield. In individual plants, SDS onset time explained 81 to 96% of the variation in final disease severity, while at the quadrat level, it explained 84 to 99% and 88 to 92% of the variation in final SDS severity and incidence, respectively. SDS onset explained 51 to 96% of the variation in number of pods, number of seeds and total seed weight on individual plants, and 84 to 95% of the variation in seed yield in quadrats. For each day SDS onset was delayed, soybean yield increased by 6.7 to 67.3 Kg/ha. This new quantitative information improves understanding of the impact of SDS on soybean yield. Further experiments are needed to determine how this relationship is affected by site-specific factors.