For food intakes to be converted into nutrient intakes a measure or estimate of the amount of food consumed is required. A number of methods have been developed to assist subjects in providing an estimate of portion size. Children's ability to use perception, conceptualisation and memory skills to estimate food portion size has not been investigated systematically. The aim of the present study was to test the effect of the timing of a dietary interview on the accuracy of estimates of food portion sizes made by children, using food photographs, food models and an interactive portion size assessment system, developed for use with children and based on portion sizes of foods consumed by children. Children (n 108) aged 4–14 years were supplied with known quantities of foods and asked to estimate the portion size of each food using each of the three portion size assessment tools. Interviews took place (a) with the food in view, (b) just after the child had eaten the food or (c) 24 h after the child had eaten the food. There were no significant differences in children's ability to estimate food portion size (either as served or as eaten) with timing of interview. That is, children were as accurate in their estimates of portion size 24 h after consuming the food as when the food was in view. Under these conditions many children were able to estimate food portion size utilising perception, conceptualisation and memory skills.