Twelve obligate heterozygotes from two kindreds were ascertained through phytosterolemic probands homozygous for molecular defects in the ATP binding cassette (ABC) half transporter, ABCG8. The response of these heterozygotes to a Step 1 diet low in fat, saturated fat, and cholesterol, and to 2.2 g daily of plant sterols (as esters) was determined in Protocol I (16 weeks) and Protocol II (28 weeks) during three consecutive feeding periods: Step 1/placebo spread; Step 1/plant sterol spread; and Step 1/placebo spread (washout). At baseline, half the heterozygotes had moderate dyslipidemia and one-third had mildly elevated campesterol and sitosterol levels. On the Step 1/placebo spread, mean LDL cholesterol decreased significantly, 11.2% in Protocol I (n = 12), and 16.0% in Protocol II (n = 7). Substitution with plant sterol spread produced a significant treatment effect on LDL levels in Protocols I and II. Conversely, the mean levels of campesterol and sitosterol increased 119% and 54%, respectively, during the use of plant sterol spread for 6 weeks in Protocol I, an effect mirrored for 12 weeks in Protocol II. During the placebo spread washouts, LDL levels increased, while those of plant sterols decreased to baseline levels in both protocols. In conclusion, phytosterolemic heterozygotes respond well to a Step 1 diet, and their response to a plant sterol ester challenge appears similar to that observed in normals.