This paper describes the characteristics of thriving and failure to thrive (FTT) children and their mothers and examines the effect of short-term lay health visitor intervention in cases of nonorganic failure to thrive (NO FTT). Twenty-five FTT children and mothers received lay health visitor (LHV) intervention in addition to other community and medical treatment; 25 other FTT children and mothers did not receive the LHV intervention but did receive all other medical and community treatment. Twenty-five thriving children and mothers were matched with the FTT children and mothers in the LHV group on the child's age at intake, sex, birth weight, and the mother's age, ethnicity, and number of living children. At initial assessment, the FTT and thriving groups were found to be comparable on demographic factors, infant birth weight percentiles, apgar scores, complications of pregnancy or delivery, and separations in the newborn period. There were more premature births in the LHV group although the proportion of premature births for the FTT and thriving groups overall were similar. A majority of mothers in the FTT groups had negative memories of childhood in contrast to more positive memories in the thriving group. At initial assessment, the majority of thriving children were developmentally normal and had increased from their birth weight percentiles whereas all of the FTT children had decreased from their birthweight percentiles and over half were developmentally delayed. There were clear differences in mother-child interaction patterns in the thriving and FTT groups. Three patterns of interaction were identified in the FTT group: benign neglect, incoordination, and overt hostility. Intervention had no measurable effect on the child's weight, development, or interaction patterns. Only 8 of 37 FTT children reevaluated 6 months later showed "catch up" growth and only 7 had improved in developmental score category. Patterns of interaction were found to persist over the 6 months in all cases. One to three year follow-up of 44 families emphasized the severity of the condition and the need for differentiation of the severity of the disturbance in the mother-child relationship and for more intensive intervention than was available in this study. Of these 44 cases, 2 children had died, 5 had been physically abused or further neglected, and 10 were in alternative care arrangements.