We undertook a retrospective pathological study of 118 skin resection specimens from 101 patients with hidradenitis suppurativa. Follicular occlusion was identified in all the specimens, regardless of disease duration (1 month to 18 years), but was not noted in the axillary and inguinal skin of controls. We therefore regard follicular occlusion as an early and important feature in the pathogenesis of the disease. The presence of apoeccrine glands in axillary skin provided an in vivo model to directly observe the effects of follicular occlusion on follicle inflammation and apocrine gland destruction. In the majority of cases, active folliculitis was associated with apocrinitis and apocrine destruction, whereas apoeccrine glands, which drain directly on to the epidermal surface, appeared intact and non-intlamed. These observations provide direct evidence in an in vivo model that follicular occlusion by keratinous material, with subsequent active folliculitis and secondary destruction of the skin adnexae and subcutis, occur as an integral step in the pathogenesis of hidradenitis suppurativa.