We have evaluated the need for daily disinfection of environmental surfaces not contaminated by biological fluids, in patient areas of a medical unit with two wings [North (N) and South (S)] at the University Hospitals of Geneva, Switzerland. Weekly bacteriological monitoring of surfaces was carried out at random (N = 1356 samples). In the S wing (control), we used detergent/disinfectant for daily cleaning of the floors and furniture. In the N wing we began by using a detergent for floors and furniture; after four weeks the results suggested changing to a rotation of detergent, dust attracting disposable dry mops and disinfectant. During this period the furniture was cleaned with an active oxygen-based compound. The average differences in contamination before and after cleaning floors were (mean reduction in bacterial counts and 95% confidence intervals; CI95): disposable mops: 92.7 cfu/24 cm2 (CI95; 74-112), active oxygen based compound 111.1 (90-133), and quaternary ammonium compound -0.6 (-27-26). Use of detergent alone was associated with a significant increase in bacterial colony counts: on average by 103.6 cfu (CI95 73-134). The quaternary ammonium compound was inadequate for disinfecting bathrooms and toilets but the active oxygen based compound was satisfactory. For furniture, there was a significant reduction in bacterial counts with both the methods using disinfectants. As the detergent was contaminated, by using it alone for cleaning, we were actually seeding surfaces with bacteria. A total of 1117 patients was studied and we observed no change in the incidence of nosocomial infections during the four months of the trial. In conclusion, uncontrolled routine disinfection of environmental surfaces does not necessarily make it safe for the patient and could seed the environment with potential pathogens.