To compare the infection rate between different durations of antibiotic prophylaxis after transrectal core biopsy and to evaluate the impact of possible risk factors. The study comprised 491 patients who underwent transrectal core biopsies of the prostate and who were randomized to receive 400 mg of norfloxacin twice daily for one day or one week. Patients receiving prophylaxis for one week had a significantly lower rate of infection (4.9%) compared to patients who received only two tablets (11%; P < 0.05). The most pronounced effect was seen in those patients with risk factors (e.g. an indwelling catheter, a former history of urinary tract infection, diabetes or prostatitis) in whom the infection rate was reduced from 17.9% to 3.3% (P < 0.02), and febrile infections from 9.5% to 1.1% (P < 0.02). Some factors have a clear impact on the risk of developing an infection after transrectal core biopsy. Prophylaxis for one week with norfloxacin is an effective way to minimize these infections.