Biofilms are microbial communities that form on a surface and are surrounded by extracellular polymeric substances. Candida biofilms are a cause of infections associated with medical devices. In the present study, an attempt was made to evaluate a significance of biofilm formation ability (BF) in virulence of C. albicans. C. albicans of 98 isolates, 24 commensal strains obtained from the oral cavities of healthy volunteers, 29 from blood culture, 25 from urine culture, and 20 from vaginal candidiasis, were assayed for BF, an ability to adhere to epithelial cells (ADH), cell surface hydrophobicity (CSH), and germ tube forming rate (GT). The relationships of BF with CSH, ADH, and GT were statistically examined. A positive correlation between BF and ADH was obtained, but the correlation (r=0.326) was relatively low. To assess BF as a factor contributing for candidiasis, mice lethality test was performed. The 10 isolates with the highest BF (mean survival rate, 24%) allow to kill mice more than those with the 10 lowest BF (mean survival rate, 47%). In addition, clinical strains isolated from blood culture, urine culture, and vaginal candidiasis showed higher BF than oral commensal strains. These results suggest BF may represent a virulent characteristic of C. albicans.