Background: Failure to thrive is generally attributed to undernutrition, but little is known about the appetite or eating behaviour of children with the condition. The hypothesis that young children who fail to thrive lack a normal sensitivity to internal hunger or satiation cues was tested in this study using an energy compensation procedure. Method: Twenty-seven children under assessment by a community-based service for failure to thrive, with weight gain in the lowest 5% for their age, were studied at one year of age with 26 controls of the same age and sex with normal weight gain, resident in the same local geographical area. Test meals were given in the child's own home on two separate days. The test meals were preceded by either a high energy (402 kJ) drink, or by a low energy (1 kJ) drink on a control day. The order was randomised, and the study conducted double blind, without the experimenter or the mother knowing which drink was which. Energy intake at the test meal was measured. Results: There was no significant difference in the birth weight of the children in the two groups but by the time of the test the cases weighed significantly less than controls, with mean (SD) weight 9.06 (1.05) kg and 11.59 (1.59) kg respectively. In relation to the British Growth Reference for weight this is a difference of 2.2 SD. Mean (SD) energy intake at the meal on the control day was significantly lower in the case children than the controls (FTT 687.5 (334.3) kJ; controls 1065.9 (431.8) kJ; p < .001). After the high energy drink, controls reduced their energy intake at the meal by a mean (SD) –257.3 (383.3) kJ while the cases showed a slight average increase of +78.1 (365.9) kJ; t=3.26, df 51, p < .001. Per kJ of the pre-load, the average change was −1.18 kJ in controls and +0.80 kJ in cases. Conclusions: The controls compensated as expected for their high energy load at the subsequent meal, but the case children did not, showing that they lack the normal responses to internal hunger/satiation cues. High energy snacks may improve the nutritional status of children who fail to thrive.