Tonsils of 20 patients with recurrent tonsillitis were investigated. Activated lymphocytes, that is lymphocytes with increased nuclear birefringence, were found in only 3 cases. The lack of lymphocyte activation is supposedly due to reduced antigen stimulation. Since antigens arrive at the tonsillar lymphoid tissue via the follicle-associated cells of the crypt epithelium, the ultrastructure of this epithelium was also studied in 5 cases. Tonsils of three dogs were used for comparison. In the repeatedly inflamed human tonsils the follicle-associated, so-called M cells contained fewer intracytoplasmic tubules and vesicles than those of the dogs. Moreover the frequency of these cells seemed to be reduced. Most of the crypts of inflamed tonsils were lined with a non-reticulated, stratified squamous epithelium. A hypothesis is presented on the pathomechanism of recurrent tonsillitis and suggestions are put forward about its pathohistology.