Background: (1,3)-β-D-glucan (BG) is a biomarker for invasive candidiasis (IC). The usefulness of BG level as a prognostic marker of treatment outcome is not well characterized., Methods: Two hundred fifty-seven patients with proven IC were enrolled in an anidulafungin study. Clinical and microbiological responses at the end of therapy were evaluated. Serial serum BG was measured. Correlation of initial and final BG levels with overall outcome was assessed in each patient., Results: Two hundred three patients had at least 2 BG levels and outcomes assessed. The majority of IC was caused by non-Candida albicans (53%) and found in the blood (84%). Overall, treatment success was 85%. In successfully treated patients, the mean ± SD initial and final BG were 573 ± 681 pg/mL and 499 ± 635 pg/mL (P = .03), respectively; while in treatment-failure patients, the levels were 1224 ± 1585 pg/mL and 1293 ± 1283 pg/mL (P = .29), respectively. A negative slope in BG levels correlated with a successful treatment outcome with a positive predictive value (PPV) of 90%, and a positive slope in BG levels correlated with treatment failure with a negative predictive value (NPV) of 90%. The cutoff value for initial BG <416 pg/mL has potential to predict treatment success with a PPV of 89%., Conclusions: A decrease in BG levels during therapy is associated with treatment success. An initial BG of <416 pg/mL has potential to predict successful treatment outcomes. Baseline and consecutive serum BG measurements may be useful as prognostic markers of treatment outcome in patients with IC receiving primarily echinocandin therapy.