Sixty patients with chronic abacterial prostatitis and 21 men without prostatitis were studied. Transperineal prostatic biopsies taken under transrectal ultrasound control were examined for antibody, complement (C3) and fibrinogen deposition using a direct immunofluorescence (IF) technique; 34 patients (57%) had prostatic tissue that displayed IF staining compared with only 1 (5%) in the non-prostatitis group. IF staining for IgM was found in 85%, for C3 in 44%, for IgA in 35% and for fibrinogen in 24%, but the IgG subclass was not detected. Antibody deposition was mainly periglandular and glandular and in the wall of vessels. Five symptoms, particularly poor urinary flow, irritative voiding and urgency, were significantly correlated with IgM and C3 deposition and, to a lesser extent, fibrinogen deposition. The aetiology of chronic abacterial prostatitis remains obscure but several possible mechanisms are discussed. The link between symptomatology and immunology could rest with functional outflow obstruction causing intraprostatic reflux of urine, this in turn inciting an immunological response, the inducing antigens being organism remnants or products, urinary constituents or autoantibodies.