Background: Hidradenitis suppurativa is traditionally classified as a disease of the apocrine gland. However, different histologic descriptions exist. Objective: Our purpose was to describe prospectively the histopathologic characteristics of hidradenitis. Methods: We systematically described and classified 60 consecutive biopsy specimens from patients with hidradenitis and compared them with 33 specimens from clinically noninvolved regional controls. Results: A heterogeneous histologic picture was found. Apocrine glands were involved in a minority of the 60 specimens, 17 showed poral occlusion, 17 simple folliculitis without poral occlusion, 9 sinus tracts, 6 epithelial cyst, 5 abscess, 3 apocrinitis, 2 diffuse dermal inflammation, and 1 pyogenic granuloma and scarring. Secondary involvement of apocrine glands was found in 12% of all specimens, and secondary involvement of eccrine glands was found in 25%. Sinus tracts were found significantly more often in the presence of poral occlusion or epithelial cysts. Control specimens frequently revealed changes compatible with early stages of follicular involvement. Apocrine glands were observed significantly more often in the axillae than in the groin. Conclusion: The clinical picture of hidradenitis suppurativa covers a broad histologic spectrum. This may help explain the therapeutic problems posed by this disease. The disease appears to be predominantly follicular, and apocrine glands appear to be primarily involved in only a minority of axillary lesions.