The growth and ageing of tonsils is described by means of the behaviour of the cell count, T cell count, DNA, and organ weight. Proliferation of cells and the influx of recirculating T lymphocytes cause hyperplasia up to the age of 17 approximately, later on the cell count and organ weight decrease again. The number of T cells in cell suspensions from the tonsil is lower than that of blood. Due to the proliferation decrease of B cells and the accumulation of recirculating T lymphocytes, the T cell count in the tonsil increases with advancing years. The tonsil function decreases as a result of this population shift. No influence of the differentiation of T cells by the tonsil can be detected.