The study presented here was conducted to determine the diagnostic value of measuring procalcitonin, C-reactive protein, and mannan antigens to distinguish fungal from bacterial infections. The sensitivity and specificity of these measurements ranged from 35% to 97%. On days 1 and 3 following the onset of fever, both serum procalcitonin and C-reactive protein levels were lower in patients with fungal infections than in those with bacterial infections (p<0.0001). The presence of mannan antigens combined with a procalcitonin level <0.5 ng/ml provided higher specificity for distinguishing fungal from bacterial infections than each result alone.