Benefits of plant sterols (PS) for cholesterol lowering are compromised by large variability in efficacy across individuals. High fractional cholesterol synthesis measured by deuterium incorporation has been associated with nonresponse to PS consumption; however, prospective studies that show this association have yet to be conducted. The goal was to test whether the lathosterol-to-cholesterol ratio (L:C ratio), a surrogate marker of endogenous cholesterol synthesis, serves as an a priori predictor of cholesterol lowering in response to PS consumption. Sixty-three mildly hypercholesterolemic adults who were preselected as possessing either high endogenous cholesterol synthesis [HS; n = 24; L:C = 2.03 ± 0.39 μmol/mmol (mean ± SD)] or low endogenous cholesterol synthesis (LS; n = 39; L:C = 0.99 ± 0.28 μmol/mmol) on the basis of baseline L:C consumed 2 g PS/d or a placebo for 28 d with the use of a dual-center, single-blind, randomized crossover design. Plasma lipid and noncholesterol sterol concentrations were measured at the end of each phase. PS consumption lowered total cholesterol (TC; -0.25 ± 0.05 mmol/L; P < 0.0001) and LDL cholesterol (-0.17 ± 0.04 mmol/L; P < 0.0001) overall. Specifically, LS individuals responded to PS treatment with a reduction in TC (-0.40 ± 0.07 mmol/L; P < 0.0001) and LDL cholesterol (-0.29 ± 0.05 mmol/L; P = 0.0002), whereas HS individuals failed to show cholesterol lowering (TC: -0.09 ± 0.09 mmol/L; P = 0.2843; LDL cholesterol: -0.05 ± 0.07 mmol/L; P = 0.4917). The odds of LS participants responding to PS consumption with cholesterol lowering better than the mean cholesterol lowering in all participants were 4.25 (95% CI: 1.242, 14.556; P = 0.0211) for TC and 3.36 (95% CI: 1.112, 10.161; P = 0.0317) for LDL cholesterol, which was higher than for HS participants. The L:C ratio predicts the extent of reduction in circulating TC and LDL cholesterol in response to PS consumption. Cholesterol synthesis assessment may thus have a use in identifying responders and nonresponders to PS therapy.