To investigate the regulation of serum levels of cholesterol precursor sterols and plant sterols, these noncholesterol sterols, fatty acids, and various parameters of cholesterol metabolism were analyzed in 63 volunteers from a randomly selected Finnish male population sample of 100 subjects, aged 50 years, who had normal dietary habits. Serum levels of cholesterol precursors, desmosterol and lathosterol (in terms of micrograms/mg cholesterol), were negatively related to both the fractional and absolute absorption of dietary cholesterol and serum high density lipoprotein (HDL) cholesterol, and positively related to overall cholesterol synthesis and serum very low density lipoprotein (VLDL) cholesterol. Serum levels of the plant sterols, campesterol and sitosterol, exhibited positive correlations with the polyunsaturated/saturated fatty acid ratio of dietary fat, the linoleic acid contents of plasma and dietary lipids, the amount of dietary plant sterols (as indicated by fecal output), fractional and absolute absorption of dietary cholesterol, and HDL cholesterol, but were inversely related to the overall cholesterol synthesis and VLDL cholesterol. Stepwise multiple regression analysis revealed that the serum level of campesterol was associated with fractional cholesterol absorption, dietary plant sterols, and biliary cholesterol secretion, and that of sitosterol with dietary plant sterols, cholesterol synthesis, fractional cholesterol absorption, and biliary cholesterol secretion. Thus, the serum non-cholesterol sterols are significant indicators of cholesterol absorption and synthesis even under basal conditions and, since gas liquid chromatographic determination of these sterols is quite simple, their measurement may be valuable for monitoring cholesterol metabolism in large-scale epidemiologic studies.