Plasma lipoprotein[a] (Lp[a]) is associated with atherogenesis and thrombogenesis. We examined how plasma Lp[a] in healthy young men was affected by fats high in stearic (C18), palmitic (C16), and lauric+myristic (C12+ C14) acid (experiment I, 15 subjects), and by fats high in myristic (C14) and palmitic (C16) acid (experiment II, 12 subjects). Strictly controlled isocaloric diets with 36% of energy from test fats were served in random order for 3 weeks separated by wash-out period(s). Diets high in C18 gave significantly higher levels of Lp[a] (51(12-560) mg/L) than diets high in C16 (38(12-533 mg/L) (P = 0.020) and C12 + C14 (34(12-534) mg/L) (P = 0.002). These differences were observed in several of the subjects in experiment I. In experiment II we saw no difference in plasma Lp[a] after diets high in C16 and C14. Our observations suggest that a fat high in stearic acid might affect Lp[a] in a different way than fats high in palmitic and myristic+lauric acid. Lp[a] concentrations were not associate with changes in tissue-plasminogen activator (t-PA) activity, factor VII coagualant activity, or plasma LDL cholesterol.