The Hartnup mutation affects an amino acid transport system of intestine and kidney used by a large group of neutral charge alpha-amino acids (six essential and several nonessential). We compared developmental outcomes and medical histories of 21 Hartnup subjects, identified through newborn screening, with those of 19 control sibs. We found no significant differences in means of growth percentiles and IQ scores between Hartnup and control groups (but all low academic performance scores were found in the Hartnup group, and various skin lesions occurred in five Hartnup subjects), no significant difference between means of the summed plasma values for amino acids affected by the Hartnup gene in Hartnup and control groups, two Hartnup subjects with clinical manifestations--impaired somatic growth and IQ in one, impaired growth and a "pellagrin" episode in the other--who had the lowest summed plasma amino acid values in the Hartnup group; the corresponding values for their sibs were the low outliers in the control group, and two tissue-specific forms of the Hartnup (transport) phenotype: renal and intestinal involvement (15 families) and renal involvement alone (one family), both forms having been inherited as autosomal recessives (the symptomatic probands had the usual form). Whereas deficient activity of the "Hartnup" transport system is monogenic, the associated plasma amino acid value (measured genotype) is polygenic. The latter describes the parameter of homeostasis and liability to disease. Cause of Hartnup disease is multifactorial.