OBJECTIVE: To determine whether premature infants with invasive Candida infection caused by strains with increased virulence properties have worse clinical outcomes than those infected with less virulent strains. STUDY DESIGN: Clinical isolates were studied from 2 populations of premature infants, those colonized with Candida spp (commensal; n = 27) and those with invasive candidiasis (n = 81). Individual isolates of C albicans and C parapsilosis were tested for virulence in 3 assays: phenotypic switching, adhesion, and cytotoxicity. Invasive isolates were considered to have enhanced virulence if detected at a level >1 SD above the mean for the commensal isolates in at least one assay. Outcomes of patients with invasive isolates with enhanced virulence were compared with those with invasive isolates lacking enhanced virulence characteristics. RESULTS: Enhanced virulence was detected in 61% of invasive isolates of C albicans and 42% of invasive isolates of C parapsilosis. All C albicans cerebrospinal fluid isolates (n = 6) and 90% of urine isolates (n = 10) had enhanced virulence, compared with 48% of blood isolates (n = 40). Infants with more virulent isolates were younger at the time of positive culture and had higher serum creatinine levels. CONCLUSION: Individual isolates of Candida species vary in their virulence properties. Strains with higher virulence are associated with certain clinical outcomes.