Amino acid excretion was investigated in 21 rachitic infants and in 22 of their parents. There was (a) increased alpha-amino acid excretion in one-third of the infants a long time after the rickets had healed, (b) an abnormally high excretion of alpha-amino nitrogen and of phosphorus in many of the parents (c) an abnormal pattern of amino acid excretion in all 9 infants tested, and (d) a good correlation between the excretion of individual amino acids by an infant and by its parents. Our findings suggest that in at least some cases of nutritional rickets there is a genetic element which may manifest itself only under adverse environmental conditions.