Introduction The risk of knee arthroplasty infection and appropriateness of antibiotic treatment are not clearly established in patients with preoperative asymptomatic bacteriuria. It has been the purpose to analyze the prevalence of preoperative asymptomatic bacteriuria in knee arthroplasty patients, as well as the incidence of prosthetic joint infection in those with asymptomatic bacteriuria treated and not with specific antibiotics. Patients and methods This prospective study included 215 consecutive knee arthroplasty patients (73 ± 6 years, 168 females) with neither urinary symptoms nor perioperative urethral catheterization. A “clean-catch” urinalysis was obtained from all patients before surgery and an urine culture if urinalysis was abnormal. Asymptomatic bacteriuria was diagnosed if >100,000 colony-forming units/ml were cultured. Patients were treated (Group A) or not (Group B) with additional specific antibiotics for urine bacteria according to surgeon criteria. Minimum follow-up reached 48 months. No patient was lost to follow-up. Results Asymptomatic bacteriuria was diagnosed in 11/215 patients (5.1 %) (11/11 females), and four of these 11 were treated with specific antibiotics (Group A). Only one patient in Group A suffered a prosthesis infection along the first 3 months (1/125, 0.5 %), but bacteria cultured from the wound were absolutely different to those in urine culture. No patient in Group B suffered a prosthesis infection. Conclusions Asymptomatic bacteriuria presents a low prevalence. We have not found any case of arthroplasty infection from urinary focus in patients with asymptomatic bacteriuria whether they received or not specific antibiotics.