The response-to-retention hypothesis in atherosclerosis states that subendothelial retention of cholesterol-rich, atherogenic lipoproteins is the central pathogenic event that is both necessary and sufficient to provoke lesion initiation in an otherwise normal artery. Sphingomyelinase-induced aggregation of low density lipoproteins (LDL) is known to facilitate LDL retention, and the only available measurements of LDL aggregates suggest LDL aggregate size is approximately 100 nm. This study investigates the hypothesis that LDL aggregate size is determined by the relative rates of sphingomyelinase hydrolysis and LDL collisions. Using a combination of dynamic light scattering and UV-vis absorbance spectroscopy to measure aggregation kinetics and particle sizes, a mass action model was developed to describe the aggregation process. It is found that LDL aggregation is sensitive to the relative amounts of sphingomyelinase and LDL and to pH. Model rate parameters were fit to experimental data in vitro and used to predict LDL aggregate sizes in vivo. The value of 100 nm in vivo does not appear to be fixed; rather, it is the value expected for the prevailing enzyme-to-LDL molar ratio.