The incidence of opportunistic fungal infections continues to increase, partly as a result of the continuing AIDS epidemic. Candida albicans remains the most important fungal pathogen and is frequently associated with oral candidiasis in HIV-infected individuals. Over the past decade, however, there has been an increasing number of reports implicating other Candida species, such as C. tropicalis, C. glabrata and C. krusei, in disease in these patients and in other patient groups. During the same period there have also been frequent reports in the literature describing what have generally been termed "atypical" C. albicans strains. These isolates have usually been recovered from symptomatic HIV-infected individuals and are unidentifiable as any recognized Candida species using conventional criteria. Two such groups of isolates recovered from cases of oral candidiasis in Irish and Australian HIV-infected and AIDS patients have been postulated to constitute a novel species which has been termed C. dubliniensis. These isolates are phenotypically very similar to C. albicans in that they produce germ tubes and chlamydospores. However, they have unusual carbohydrate assimilation patterns and grow poorly or not at all at 42 degrees C. Using a variety of DNA fingerprinting techniques and karyotype analysis, the genomic organization of C. dubliniensis was shown to be distinctly different from that of C. albicans. Classification of C. dubliniensis as a separate species was confirmed by phylogenetic analysis, whereby the comparison of ribosomal RNA sequences demonstrated that C. dubliniensis isolates formed a cluster clearly distinct from other Candida species, including C. albicans, to which it is most closely related. Since its original identification, atypical Candida isolates from around the world have been positively identified as belonging to this species. To date, isolates of C. dubliniensis have been recovered mainly from the oral cavities of HIV-infected individuals and are most frequently implicated in cases of recurrent infection following antifungal drug treatment. The clinical importance of this species and the role of drug resistance in its epidemiology have yet to be determined.