Patients with chronic prostatitis have an increased number of white blood cells in expressed prostatic secretion. Two groups can be separated, one is characterized by uropathogenic bacteria in expressed prostatic secretion and recurrent urinary tract infections, chronic bacterial prostatitis. In this group an immune response to the bacteria has been demonstrated. Patients belonging to the other group, non-bacterial prostatitis, have similar symptoms. Many harbour Gram-positive bacteria in a high number, often Staphylococcus epidermidis in expressed prostatic secretion. This bacteria is usually not considered in prostatitis in spite of extreme high numbers. The etiology of non-bacterial inflammations is thus unknown. Forty-three per cent of the patients with chronic prostatitis had Gram-positive bacteria and 13% had Gram-negative in expressed prostatic secretion. Forty-four per cent of patients referred with symptoms of prostatitis did not have any aerobic bacteria at the prostatic level in sufficient number for the diagnosis bacterial prostatitis according to Meares and Stamey and form thus a third group. Antibiotic treatment of patients with non-bacterial prostatitis reduced symptoms but also changed the bacterial flora in urethral and prostatic secretion in such a way that uropathogens were found after treatment. In a group of patients an immunologic response to Staph. epidermidis was searched for by measuring complement components (C3c, C4c) as well as ceruloplasmin in serum and immunoglobulins (IgA, IgG) in seminal plasma. A specific ELISA method to estimate antibodies in serum against Staph. epidermidis was tested. No specific pattern separated patients from controls or patients with Gram-negative bacteria from patients with Gram-positive bacteria. Staphylococcus saprophyticus in cultures from men with prostatitis were more frequent in the third quarter of the year. The bacteria seemed to appear during or after antibiotic treatment but disappeared spontaneously during a follow-up period of six months. Treatment with the surfactant sodium pentosanpolysulphate, a heparinoid, given orally to patients with chronic prostatitis reduced concomitant pain in muscles and joints. The possibility of an altered host factor function in the polymorphonuclear leucocytes of patients with chronic non-bacterial prostatitis colonized with Staph. epidermidis was investigated. Chemotaxis, phagocytosis and intracellular killing were reduced in vitro and may to a part explain the bacteriological findings. Careful evaluations must thus be performed of earlier neglected factors to reach better knowledge of the chronic prostatitis.