Background: Bacteria have been suggested as a possible cause of hidradenitis. Different species have been found in several small studies, and particularly the presence of Streptococcus milleri has been linked to disease activity. Objective: To investigate the possible role of bacteria in hidradenitis. Methods: Cultures of active lesions and serological analysis of circulating IgG antibodies were studied in 41 patients. Results: Bacteria were found in 49% (20/41) of all lesions: Staphylococcus aureus, 8; S. milleri, 1; Staphylococcus epidermidis, 11; Staphylococcus hominis, 1, Corynebacterium spp., Acinetobacter and Lactobacillus spp. were found once each and considered as contaminants. Patients in whom S. aureus was found had a shorter duration of disease (mean 1.7 vs. 9.7 years for sterile lesions). No other significant correlations were found between location of disease or antibody response and the bacterial species found. Conclusion: S. milleri appears to be an unusual pathogen, and bacteria are only found in approximately 50% of all active hidradenitis lesions. It is suggested that S. aureus may play a temporary role in the early phase of the disease, but additional longitudinal studies of cohorts of patients are needed to clarify this point.