Three male patients with a clinical history of prostatitis with coagulase-negative staphylococci localized to the expressed prostatic secretion and who did not respond to antibiotics were studied intensively 4 weeks after cessation of therapy with repeat culture of the prostatic fluid, as well as with culture, and histological and ultrastructural examination of multiple prostatic biopsies. Coagulase-negative staphylococci were cultured in the biopsied prostatic tissue, and gram-positive staphylococci were identified in sparse and focal microcolonies adherent to the prostatic ductal walls. Coagulase-negative staphylococci may be implicated in the pathogenesis of chronic bacterial prostatitis.