• To determine if subjects with phenylketonuria receiving diets significantly lower in cholesterol and saturated fat had serum lipid concentrations different from those of their family members, we measured serum concentrations of total cholesterol, high-density lipoprotein cholesterol, and total triglycerides in the probands with phenylketonuria, their parents, and their siblings. Eleven adults (seven women and four men) and 16 children (eight girls and eight boys) were studied. Ten subjects (four girls and six boys) had phenylketonuria. Subjects with phenylketonuria consumed less cholesterol (0.02 vs 0.41 mmol/d ) and fat (median, 21% vs 39.5% of total calories), and their diets had a higher ratio of polyunsaturated to saturated fatty acids (median, 2.0 vs 0.2) than did their siblings without phenylketonuria. The diet of the parents was similar to that of their offspring without phenylketonuria. No differences were noted between the subjects with phenylketonuria (consuming a diet lower in saturated fat and cholesterol) and their siblings without phenylketonuria in serum concentrations of total cholesterol (median, 3.34 vs 3.07 mmol/L); high-density lipoprotein cholesterol (median, 1.44 vs 1.37 mmol/L); low-density lipoprotein cholesterol (median, 1.44 vs 1.09 mmol/L); or triglycerides (median, 0.89 vs 0.54 mmol/L). We conclude that previously reported lipoprotein abnormalities noted between unrelated subjects with and without phenylketonuria may not be due to differences in dietary intake, but rather due to a (genetic) predisposition of the population with phenylketonuria toward lower serum lipid concentrations.