The thymus in the paediatric chest will often give radiological appearances of very intriguing nature. It is known to react dramatically to stress factors such as infection or trauma. Gross hypertrophy following atrophy has been documented in many cases. In this paper the author scrutinizes one hundred anteroposterior chest radiographs of children under the age of five years suspected of having chest infection. Thymocardiac ratio is determined. It is concluded that in children under five years, the thymus generally decreases in size with age and that often the thymus will undergo atrophy as a primary response to infection.