Previous studies of middle-aged men have shown a univariate association between low density lipoprotein (LDL) particle diameter (size) and coronary heart disease, but this association has yet to be examined in younger women. Using a subsample from a population-based case-control study of women living in western Washington State, the authors examined the association between LDL particle size and risk of early-onset myocardial infarction (MI) in 1992-1995. Gradient gel electrophoresis was used to characterize LDL subclasses in nonfasting blood samples from 72 MI cases and 159 controls aged 20-44 years. Mean LDL particle size in cases was significantly smaller compared with controls (26.4 vs. 26.9 nm, p < 0.001), with an odds ratio of 2.3 (p < 0.0001) for a 1-nm smaller LDL particle size. These results were independent of age, menopausal status, smoking, diabetes, hypertension, and LDL cholesterol (odds ratios = 1.9-2.3 for a 1-nm smaller LDL particle size, all p < 0.02) but were not independent of body mass index, high density lipoprotein cholesterol, or triglyceride (odds ratios = 1.4, 1.4, and 1.1, respectively; all p > 0.05). Therefore, in age-adjusted analyses, smaller LDL particle size was associated with MI in young women, but the risk was attenuated after adjustments for metabolic factors related to both LDL particle size and MI.