The histological features of 149 trichilemmal cysts removed from sixty-five patients are reviewed and related to the clinical findings. These cysts, which may be solitary or multiple, gradually increase in size and number, and may produce daughter cysts by budding. When a breach occurs in the wall, inflammatory cells pour in but do not replace the cyst wall. This event may be followed by healing from the margins of the breach, by marsupialization to the overlying epidermis and thus natural resolution, or by proliferation to produce a pseudo-epitheliomatous change which can be confused with a well differentiated squamous cell carcinoma.

SUMMARY On the basis of clinical, histological and electron microscopical findings, we are proposing that mycosis fungoides can begin in the epidermis. Early involvement of the basal layer of the epidermis is emphasized by the pigmentary changes which herald the onset of about half the cases. The formation of new fibrous tissue above the subepidermal elastic garland and the presence of PAS‐positive globules above the epidermal basement membrane, support the view that the primary change is on the epidermal side of the elastic garland and basement membrane. The predominantly epidermal distribution of the mycosis cell in many early cases suggests that it stems from a cell normally present in the epidermis. The likeliest candidate appears to be the Langerhans cell. A derivation from this cell would explain many puzzling features of mycosis fungoides and in particular the affinity of the mycosis cell for the epidermis.

Summary One hundred and fifteen individuals from 60 families are described who had trichilemmal cysts. These were solitary or multiple intradermal cysts and occurred most frequently on the scalp. A punctum leading from the skin surface to the cyst was not seen in any patient. Because they are encapsulated they are easily removed. Histologicaily they resemble the external root sheath in the region of the follicular isthmus and are quite different from epidermoid cysts. They are not associated with Gardner's syndrome or any other diseases. In 46 of the 60 families an autosomal dominant mode of inheritance was demonstrated.