We studied the interrelationship of diet and plant sterols (PS) on plasma lipids, lipoproteins and carotenoids. Mildly hypercholesterolemic men (n = 13) and postmenopausal women (n = 9) underwent four randomized, crossover, double-blind, controlled feeding periods of 23 days each. The design consisted of two levels of PS (0 and 3.3 g/day) and two background diets having fat content either typical of the American diet (total and saturated fat at 33.5 and 13.2% of energy, respectively), or a Step 1 type of diet (total and saturated fat at 26.4 and 7.7% of energy, respectively). Plasma total cholesterol (TC), high density lipoprotein (HDL) cholesterol, low density lipoprotein (LDL) cholesterol, Apo A1 and Apo B were 4.3, 5.3, 4.5, 2.8 and 2.5% lower, respectively (P <or= 0.0001; <0.0001, 0.0016, 0.0006, and 0.0069), with the Step 1 diet than with the typical American diet. Diet had no effect on TC/HDL cholesterol (P = 0.1062). Plant sterol intake lowered TC, LDL cholesterol, and Apo B by 9.0, 12.4 and 6.1% and TC/HDLC by 9.6% (P <or= 0.0001 for all), respectively, without affecting HDL cholesterol and Apo A1 (P = 0.2831 and 0.732). The PS effect in lowering plasma TC and LDL cholesterol was independent of and additive to the effect due to dietary fat reduction. Responses of plasma carotenoids to PS intake were consistent with the literature.