The aim of the study was to determine the effect of replacing saturated fatty acids (SAFA) with polyunsaturated fatty acids (PUFA) in the diet on plasma lathosterol concentration, an index of cholesterol synthesis, in mildly hypercholesterolaemic subjects.The study was a randomised, cross-over trial.Seventy-six subjects were recruited and one dropped out. Twenty-nine men and 46 women with baseline plasma cholesterol levels in the range 5.5-7.9 mmol/l were entered into the trial.Subjects were placed on two isocaloric diets, one rich in SAFA and the other high in PUFA, for 6 weeks each. Plasma lathosterol and lipid and lipoprotein concentrations were measured.When PUFA replaced SAFA in the diet, there was a significant (P = 0.01) fall in plasma lathosterol concentration in all subjects combined [-1.37 (-0.36 to -2.38) mumol/l] which was largely due to a significant (P = 0.03) decrease in men [-2.35 (-0.42 to -4.28) mumol/l] but not in women. The concomitant decreases in plasma cholesterol, low density lipoprotein (LDL) cholesterol and high density lipoprotein (HDL) cholesterol were similar in men and women. Plasma lathosterol concentration and the ratio lathosterol/cholesterol were significantly higher in men at baseline and during diets rich in SAFA but were not significantly different between the sexes during diets rich in PUFA.These data suggest that replacing SAFA with PUFA in the diet may reduce cholesterol synthesis in mildly hypercholesterolaemic men and markedly reduce the gender-related difference in cholesterol synthesis commonly observed.