Lower low-density lipoprotein (LDL) peak diameter and a predominance of small, dense LDL are associated with type 2 diabetes, but it is unclear whether they are a risk factor for gestational diabetes mellitus (GDM). To evaluate whether prepregnancy lipid profile predicts the development of GDM during pregnancy. A nested case-control study among women who participated in a multiphasic health exam, where blood was collected and stored between 1984 and 1996, and who then had a subsequent pregnancy between 1984 and 2009. Kaiser Permanente Northern California. Cases were 254 women who developed GDM. Two controls were selected for each case and matched for year of blood draw, age at baseline, age at pregnancy, and number of intervening pregnancies. Prepregnancy LDL peak diameter and prepregnancy lipid subfraction concentrations grouped according to size, and the odds of developing GDM. Women in the lowest quartiles of LDL peak diameter and high-density lipoprotein had increased odds of GDM compared with women in the highest quartiles (odds ratio [95% CI], 2.60 [1.37-4.94] and 1.98 [1.01-3.86], respectively), in multivariable adjusted models. Being in the highest quartile of small and very small LDL subfractions also increased the odds of GDM (2.61 [1.35-5.03] and 2.44 [1.22-4.85], respectively). Lower LDL peak diameter size and high-density lipoprotein levels and higher levels of small and very small LDL subfraction groups were present years before pregnancy in women who developed GDM. A prepregnancy atherogenic lipid profile may help identify women at risk of GDM to target for prevention.