The authors analyse preoperative preparation of 91 patients with benign prostatic hyperplasia (BPH). Two groups of patients received conventional preparation (group 1) and magnetotherapy (group 2) before TUR of the prostate. The examination covered immune system, bacteriological indices of urine and prostatic tissue. Infection of the urinary tract is a main risk factor of complications after TUR. Conventional preoperative preparation fails to correct immunity, to change bacterial urine flora, to improve hemodynamics in the prostate. Transrectal magnetotherapy with running magnetic field eliminates deficiency of T- and B-cell immunity, raises functional activity of B-lymphocytes and phagocytic ability of neutrophils, reduces endogenic intoxication, tissue edema, bacterial contamination, number of thrombohemorrhagic complications. This leads to a decrease in the number of postoperative complications.