Candida tropicalis has been known to be a major cause of invasive Candida infection. Numerous reports have documented C. tropicalis as the most common species of Candida other than C. albicans. The epidemiology and antifungal susceptibility of C. tropicalis are poorly defined. A series of 89 clinical isolates of C. tropicalis from 56 patients hospitalized at seven different U.S. medical centers were analyzed by restriction endonuclease analysis of genomic DNA (REAG) using the restriction enzymes Sfil and BssHII followed by pulsed-field gel electrophoresis (PFGE). The MICs of the isolates for amphotericin B, 5-fluorocytosine (5FC), fluconazole, itraconazole, and D0870 were determined by microbroth dilution testing. A total of 49 different DNA types were identified among the 89 isolates. Generally, each DNA type represented an individual patient, and serial isolates from the same patient were the same DNA type. Small clusters of patients infected with the same DNA type of C. tropicalis suggested possible nosocomial transmission. The MICs of the various antifungal agents were amphotericin B 0.5 to 2.0 micrograms/ml (MIC90 = 2.0 micrograms/ml), 5FC 0.25 to 1.0 microgram/ml (MIC90 = 0.5 microgram/ml), fluconazole 0.25 to 8.0 micrograms/ml (MIC90 = 1.0 microgram/ml), itraconazole 0.03 to 1.0 microgram/ml (MIC90 = 0.5 microgram/ml), and D0870 0.007 to 0.12 microgram/ml (MIC90 = 0.03 microgram/ml). These data support previous observations that infections caused by C. tropicalis frequently originate from the patient's own endogenous flora. Clusters of a single strain in individual hospitals also suggests that limited nosocomial transmission may occur.