The apparent increase in the prevalence of candidal infections among patients with cancer and the changes in the spectrum of Candida pathogens infecting these patients are associated with a number of factors. Selection of less-susceptible organisms occurs because antibacterial and antifungal agents alter microbial flora, and the use of these agents can lead to mutations that confer antibiotic resistance to initially susceptible organisms. Candida organisms that are resistant to polyenes and azoles have been increasingly isolated from specimens from oncology patients, and outbreaks of infections due to these less-susceptible organisms have been reported in several centers. Antifungal resistance has largely been observed among non-albicans Candida species, especially Candida parapsilosis, Candida lusitaniae, and Candida krusei. However, there also has been an increase in the isolation of resistant Candida albicans, the more prevalent and more virulent species of Candida, from specimens from other patients. Isolation of resistant C. albicans from oncology patients has been rare, although the potential for further problems exists. These observations emphasize the importance of the selective pressure exerted by antifungal agents on the microbiological flora of patients and the influence of these agents on the prevalence and spectrum of fungal pathogens in clinical settings.