Background: Epidermal cyst is a common acquired skin cyst. When such cysts may be inflamed, they are often referred to as being infected. To clarify the etiology of inflamed epidermal cysts, several studies have carried bacteriology of inflamed and uninflamed epidermal cyst and sought to identify the role of micro-organisms. Objective: The aim of this study is to evaluate the bacterial influences on inflammatory process of epidermal cysts and the antibiotic sensitivity of cultured aerobic bacteria in epidermal cysts. Methods: We carried out the bacterial cultures using sterile swabbing internal materials in each cyst which has been incised after surgical removals of epidermal cyst. An inflamed epidermal cyst was defined as a known cyst that developed a fluctuant soft-tissue swelling surrounded by the erythema and contained a localized collection of purulent material. For the aerobic culture, samples were cultured on blood agar plates, McConkey agar plate and chocolate agar plates in 5% CO2 at 35oC for 5 days. Colonies formed were identified based on VITEK2 system. Then antimicrobial susceptibility test were also done on VITEK2 system. Results: Total of 100 epidermal cyst specimens from 96 patients (67 men and 29 women) were involved had confirmed histopathological findings by dermatologists. Seventy were from face and neck, 24 from trunk, 4 from the extremities, 2 from buttock. Of the 53 inflamed cysts, 30 (56.6%) yielded bacterial growth. On the other hand, from the 47 uninflamed cysts, 15 (31.9%) cyst resulted in bacterial growth (p=0.013). The predominant bacteria from inflamed and uninflamed cyst were coagulase-negative Staphylococci (32 isolate of 45 specimens, 71.1%). All cultured bacteria were susceptible to almost all of antibiotics except amoxicillin/clavulanic acid, ampicillin, benzylpenicillin, cefoxitin, cephalothin, fucidic acid, piperacillin, piperacillin/Tazobactam. Conclusion: Our study showed that CoNS was predominant in inflamed cysts, which strongly suggests that aerobic bacteria play a role in the inflammatory process and treatment with antibiotics is necessary for epidermal cyst.