In order to investigate the epidemiology of colonization and possible transmission of yeasts among patients and healthcare workers in adult intensive care units (ICUs), 194 patients were followed for a mean of 9 +/- 11 days and 63 healthcare workers were followed for a mean of 132 +/- 52 days. Among the patients, 142 (73%) were colonized by yeast, with Candida albicans being the species most commonly recovered. Most patients (65%) were already colonized with yeast upon admission to the intensive care unit; only 17% became colonized after admission. Persistent colonization occurred in 51 (55%) of 92 patients who had more than three cultures performed; in 75% of them, colonization persisted with the same strain of Candida albicans or Candida glabrata. Bacterial infection in the month preceding entry into the ICU was the only risk factor significantly associated with yeast colonization. Among the healthcare workers, yeasts were isolated from 42 (67%). Candida albicans was most frequently recovered from the oropharynx (19% of occasions), and Candida parapsilosis was most frequently found on hands (8% of occasions). Persistent colonization of the oropharynx occurred in only six healthcare workers, and none had persistence of yeasts on hands. In this non-outbreak setting, 5 (4%) of 123 patient/healthcare worker interactions that were linked epidemiologically yielded the same strain of Candida albicans, providing evidence for possible cross-transmission. No similar link was found between healthcare worker-patient interactions and colonization with Candida glabrata or Candida parapsilosis.