We investigate the relation between LDL size and LDL-composition, lipid profile and insulin sensitivity on an early step in young, metabolically healthy.Methods: 51 young, metabolically healthy subjects are enrolled in the study (age (y) 23.6 ± 2.4; BMI 23.7 ± 3.4 kg/m 2 , RR-syst. 119 ± 11, RR-diast. 78 ± 9). Fasting insulin, C-peptide, TG, VLDL-TG, HDL-C, LDL-composition (C, FC, CE, TG) and CETP activity are measured. LDL-size is estimated by PAGGE. Insulin sensitivity is assessed by the hyperinsulinemic euglycemic clamp test.Results: LDL size is significantly related to total TG (r = -0.68, p < 0.000), LDL-TG/apoB ratio (-0.27, p = 0.05), LDL-FC/apoB ratio (r = 0.45, p < 0.02), C-peptide (r = 0.44; p < 0.02) and insulin sensitivity (r = 0.62, p < 0.000). No relation is found to BMI, RR, HDL-C and CETP activity. In a stepwise regression analysis, TG, C-peptide and glucose utilisation rate are the major determinants for LDL-size variability (p < 0.02, respectively). If insulin resistance is set to a cut-off of 35 mg/kg (1), significant differences were found between insulin sensitive (IS; > 35 mg/kg, n = 35) and insulin resistant (IR; <35 mg/kg; n = 16) subjects: in the IR group, BMI, C-peptide, TG and VLDL-TG were significantly increased (p < 0.02, respectively) and LDL size decreased (p < 0.01) whereas RR, HDL-C, LDL-C and CETP activity did not differ.Conclusions: In young, metabolically healthy subjects, LDL size was determined by glucose utilisation rate and TG. Therefore, smaller particles were observed in healthy subjects with an impaired insulin sensitivity, increased TG and fasting C-peptide levels.