The pathogenesis of hidradenitis suppurativa (HS) has given rise to controversy about whether the central pathogenetic feature is an apocrine follicular occlusion (with subsequent bacterial infection) or it is a folliculitis with secondary involvement of both apocrine and eccrine sweat glands. Most previous concepts have focused on apocrine gland involvement. A prospective study of 27 consecutive patients (39 biopsies) was performed to investigate the immune-histological findings in early, newly formed lesions. Most of them were examined within 3 days of clinical onset. Histopathological findings showed that the initial lesion is an occluding spongiform infundibulo-folliculitis. Furthermore, it was found that T cells predominated in the lymphocytic cell population. A high percentage of HLA-DR positive lymphocytes was found in an inverse relationship with Leu-8 positive lymphocytes. Additionally, a sharp decline in the T-helper/suppressor ratio was observed after the initiation. These results reveal that the primary event in HS is an infundibulofolliculitis with secondary involvement of apocrine glands. Acne inversa would be a more appropriate name.