To assess relationships of serum phytosterols (plant sterols [P]) to serum cholesterol (C), P were measured by gas-liquid chromatography (GLC) in 595 hypercholesterolemics (top C quintile in screening of 3,472 self-referred subjects). A second specific aim was to determine whether high serum P would track over time and whether they would predict familial aggregation of high C, high low-density lipoprotein cholesterol (LDLC), high apolipoprotein (apo) B, and increased premature coronary heart disease (CHD) in hyperphytosterolemic probands and their first-degree relatives. Mean +/- (SD) C was 260 +/- 56 mg/dL, campesterol (CAMP) was 2.10 +/- 1.6 micrograms/mL, stigmasterol (STIG) 1.71 +/- 1.67, sitosterol (SIT) 2.98 +/- 1.61, and total P 6.79 +/- 3.66 micrograms/mL. Serum C correlated with CAMP (r = .15, P less than or equal to .001), STIG (r = .10, P less than or equal to .02), SIT (r = .34, P less than or equal to .0001), and total P (r = .29, P less than or equal to .0001). High serum CAMP and STIG were associated with a personal or family history of CHD in subjects less than or equal to age 55 years (premature CHD). In 21 hyperphytosterolemic probands who initially had at least one P at or above the 95th percentile and a second P at or above the 75th percentile, P were remeasured 2 years later. Initial and 2-year follow-up CAMP, STIG, and SIT did not differ (P greater than .7). Initial and follow-up CAMP were correlated (r = .47, P = .03).(ABSTRACT TRUNCATED AT 250 WORDS)