Background: Prurigo nodularis is an eruption of lichenified or excoriated nodules caused by intractable pruritus that is difficult to treat. Therefore the antipruritic efficacy of capsaicin seemed to be of particular interest because this alkaloid, extractable from red pepper, interferes with the perception of pruritus and pain by depletion of neuropeptides in small sensory cutaneous nerves. Objective: The aim of this concentration- and regimen-ranging study was to evaluate the efficacy, safety, and practicability of capsaicin in the topical treatment of prurigo nodularis in a large series of patients. Methods: A total of 33 patients with prurigo nodularis of various origins were selected to receive capsaicin (0.025% to 0.3%) 4 to 6 times daily for 2 weeks up to 10 months. The consecutive follow-up period was up to 6 months. In 7 patients, skin biopsy specimens were taken before, during, and after therapy and investigated histologically, immunohistochemically, and ultrastructurally. Results: All 33 patients could be evaluated for efficacy. After cessation of the symptoms of neurogenic inflammation, such as burning sensations or erythema, all of them experienced a complete elimination of pruritus within 12 days. In addition, capsaicin largely contributed to the gradual healing of the skin lesions. After discontinuation of the therapy, pruritus returned in 16 of 33 patients within 2 months. At the ultrastructural level, no degenerative changes of cutaneous nerves could be found during or after capsaicin therapy. Depletion of substance P was demonstrated by confocal laser scanning microscopy thus confirming the specific effect of capsaicin in vivo. Conclusion: Topical treatment of prurigo nodularis with capsaicin proved to be an effective and safe regimen resulting in clearing of the skin lesions. (J Am Acad Dermatol 2001;44:471-8.)