Immunological deficiency is seldom considered in the pathogenesis of chronic prostatitis, despite clinical symptoms of prostatic inflammation and occasionally also in other tissues. Investigations in three patients with severe bacterial prostatitis with Gram-positive bacteria in the prostatic secretion revealed a decreased phagocytotic activity of the polymorphonuclear leukocytes (PMN-cells) derived from patients' sera towards the autologous Gram-positive bacteria from their own prostatic secretions but not towards heterologous Gram-positive bacteria from other patients or controls. These observations indicate a hitherto unobserved, altered host-parasite interaction in patients with prostatitis possibly caused by Gram-positive bacteria.