Differentiation between organic and nonorganic failure to thrive (FTT) often requires prolonged, complicated hospital evaluation. This study was aimed at developing a more direct diagnostic method for distinguishing hospitalized organic from nonorganic FTT infants on the basis of quantifiable social behaviors. An easily administered 1- to 7-point approach-withdrawal scale was created to monitor daily a standardized sequence of brief social interactions between examiner and infant. Behavioral profiles of three matched groups of patients aged 6 to 16 months were compared: eight infants with nonorganic FTT, ten infants with organic FTT, and seven normally grown infants hospitalized for medical reasons. An underlying behavioral pattern systematically and statistically distinguished nonorganic FTT infants from organic FTT infants and the control patients, despite the enormous variability in illness and social distortions in all three groups. Infants with nonorganic FTT predictably prefer distant social encounters and inanimate objects, while organic FTT infants and medically ill control patients consistently respond most positively to close, personal interactions such as touching and holding. Aside from helping to differentiate types of FTT, this study provides a reliable and clinically useful observational tool for easily monitoring ongoing socioemotional status of hospitalized infants.