The effectiveness of a clinical and a microbiological follow-up systems for the detection of hospital-acquired infection (HI), both independently and simultaneously applied, were prospectively evaluated. The observed incidence rate was 11.4%. The clinical follow-up detected 81.3% of HI while microbiological follow-up detected 59.7%. This difference was statistically significant (p less than 0.001). It took 43 and 45 minutes to detect each instance of HI with the clinical and microbiological methods, respectively. Clinical methods are the most adequate to obtain maximal sensitivity in the overall surveillance of hospital-acquired infection, whereas microbiological follow-up may be useful for the detection of some types of HI such as bacteremia or urinary tract infection.