Underlying illness associated with failure to thrive in breastfed infants. Over a four-year period in a suburban pediatric practice, 38 infants aged six months or less were identified with failure to thrive (FTT) while breast-feeding. In seven cases (18.4%), an associated organic illness was diagnosed. Only 2 of 28 breast-fed neonates (8%) were found to have FTT associated with another illness, as compared to 5 of 10 older infants (50%). Breast-fed infants with FTT, particularly those presenting after the first month of life, should be considered high risk for having other disease. Their clinical evaluation should include an appropriate search for organic illnesses. 