Aims:  The objective of the study was to determine the microbiological quality of samples of water and dialysate in a haemodialysis unit. Methods and Results:  Seventy‐two samples each of water and dialysate were collected during November 2003 to April 2004. The following microbiological analyses were performed: test for total and faecal coliforms, which produced negative results for all the samples; counts of total heterotrophic bacteria, where three samples of water and two of dialysate showed levels higher than those permitted by national standards; and endotoxin assay, which revealed high quantities only in samples of water that preceded reverse osmosis. Nonfermenting Gram‐negative bacteria were identified in 54 samples of dialysate and in 26 samples of water. The test for adhesion to an inert surface showed that various bacteria were capable of forming biofilms. Twenty‐seven per cent of the bacteria were resistant to sodium hypochlorite at 500 ppm for 10‐min contact time. Sixty per cent of the isolates were resistant to three or more antibiotics. Conclusions:  Water and dialysate can be a source of infection for patients who need haemodialysis. Significance and Impact of the Study:  An adequate system for water treatment, disinfection of the haemodialysis system and microbiological monitoring of the water and dialysate are necessary to reduce bacteraemia and pyrogenia outbreaks.