The cause of nonorganic failure to thrive (NFT) was studied prospectively in 274 low-income pregnant mothers by interviewing them for characteristics that retrospective studies indicated were associated with child maltreatment. After delivery, medical charts of mothers and infants were reviewed for complications of pregnancy, delivery, and postdelivery hospitalization, and the infants were observed for growth failure. The characteristics of 15 families with infants who developed NFT were compared with those of 86 selected randomly from the remaining families. Interview results suggest that NFT correlates significantly with aberrant nurture during the mother's childhood and with conflicts between the parents of the infants. Perinatal events correlating significantly with NFT include complications of pregnancy, short gestation, and residual minor medical problems at discharge from the nursery. We speculate that these events predispose to NFT by disturbing the development of the mother-infant relationship.