Twenty-one infants with failure to thrive were examined, shortly after hospital admission, for a set of eight noninterpersonal and six interpersonal behaviors identified as possibly being associated with nonorganic failure to thrive. The infants selected for study exhibited some of the target behaviors early in the course of hospitalization, and all were subsequently diagnosed as having nonorganic failure to thrive. The target behaviors include inactivity; irritability; posturing; lack of affect; rumination; excessive thumb-sucking; disproportionate use of hands and fingers rather than arms, legs, and trunk; crying when approached; lack of or decreased vocalization; lack of cuddling; poor eye contact; lack of response to a human stimulus; and indifference to separation. Abnormal interpersonal behaviors were more common than abnormal noninterpersonal behaviors. Identification of these target behaviors may be valuable in diagnosing nonorganic failure to thrive, avoiding the necessity for expensive organic evaluations and providing for earlier social assessment and intervention.