Colonization of patients and nursing staff on an intensive care unit by Candida albicans was studied over a 4-month period. Multiple swabs and samples were taken from patients and their nurses on 12 occasions during the study period. During this study there were no obvious clinically relevant candidal infections. Patients yielded C. albicans from at least one body site on 68%, and nurses on 57% of occasions. All isolates of C. albicans were further characterized by both morphotyping and resistotyping. All patients but one were colonized by a single strain throughout their stay on the unit, whereas nurses were often colonized by more than one strain type. Strains isolated from nurses' hands were all indistinguishable from strains colonizing the patient under their care, whilst strains isolated from nurses' mouths were usually distinct from their patients'. The probability that a C. albicans culture positive nurse carried an indistinguishable strain from that of her positive patient was p = 0·632, compared with p = 0·325 that a randomly chosen positive nurse would carry a strain indistinguishable from that of a randomly chosen positive patient. Diversity in patients' strains was low (D = 0·667) compared with nurses' strains (D = 0·778). These results suggest that cross-infection by C. albicans is a common occurrence on intensive care units. The reason outbreaks of candidal disease are not more common may be related to differences in virulence of the strains.