The relationship between chronological age, conservation ability and concepts of illness was investigated in 5- to 7-year old primary school children. Four conservation tasks were used: conservation of number, substance, length and liquid. The children's concepts of illness were assessed according to Bibace & Walsh's scoring system, which assumes Piaget's stages of cognitive development. The analysis of the data showed that the number of concrete-logical explanations varied both with chronological age and conservation ability. However, the number of pre-logical explanations of illness stayed constant over different age groups and over different levels of conservation ability. The increased number of concrete-logical explanations by the older children was due to the fact that more questions were answered by these children. It was concluded that changes in the understanding of illness are better explained by an increase in information than by a qualitative shift from one cognitive stage to another.