Group A Streptococcus has been identified as a possible etiologic agent in psoriasis in epidemiologic, immunologic, immunopathologic, medical, and surgical studies. Tonsillectomy has been shown to provide considerable relief to 75% of patients with plaque psoriasis. Even with the substantial evidence supporting group A Streptococcus as a causative pathogen in psoriasis, it is an elusive pathogen because it is not culturable, nor does it exhibit any positive serologic evidence of its presence. One possible reason for the negative cultures and negative serology findings with group A Streptococcus is the development of biofilms. We conducted a pathologic study to determine whether biofilms were present in the tonsillar tissues of 10 patients with psoriasis-6 men and 4 women, aged 25 to 64 years (mean: 48)-and in 10 age- and sex-matched controls with chronic tonsillitis who did not have psoriasis. We found that biofilms were present in every tonsillectomy specimen we examined, including those of the controls. Whereas psoriasis has been considered a "double hit" phenomenon, we believe that the development of skin lesions is likely attributable to the presence of the gene PSORS together with the biofilm in psoriasis patients rather than to the biofilm itself. Biofilms have been identified in both extra- and intracellular locations. We believe our findings add further evidence supporting a microbial pathogenesis of this disease.