The effects of chlorhexidine mouthrinses, used as a supplement to mechanical oral hygiene measures, were studied in patients receiving treatment for acute leukemia. Twenty-eight patients were randomly divided into two groups. During two periods, when the patients were taking medication for the leukemia, one group rinsed with a 0.2% chlorhexidine solution twice daily and the other group did not. Chlorhexidine had no effects of any clinical significance on parameters such as number of days with fever, number of oral lesions, plaque score, gingival bleeding score, or occurrence of candidiasis. There was, however, an increased number of patients who had a burning sensation in the mouth, and a tendency toward increased numbers of salivary enterococci, enterobacteria, and/or Pseudomonas in patients who rinsed with chlorhexidine. The results of the present study do not support the use of chlorhexidine mouthrinses in patients who are able to maintain good oral hygiene by mechanical means during their illness.