BACKGROUND Eosinophilic pustular folliculitis (EPF) was initially defined as a sterile folliculitis of unknown cause. Because attempts to demonstrate bacterial organisms have been unsuccessful, and antibiotic therapy is usually ineffective, a bacterial infection is not considered a plausible causative factor for this disease. OBJECTIVE Our purpose was to describe five patients with the clinical and histologic characteristics of EPF and to report the results of bacterial cultures. METHODS Biopsy specimens were examined and pustules were cultured. RESULTS In three of the five patients, Pseudomonas infection of the hair follicle was the cause of the disease as proven by repeated cultures and the response to specific therapy. Three patients had a systemic disorder known to cause immunologic alteration: AIDS in one and a myeloproliferative disorder in two. CONCLUSIONS Although EPF was initially defined as a sterile folliculitis of unknown origin, three of our patients had an identifiable and treatable cause. We believe that these cases warrant the diagnosis of EPF.