Echinocandins, such as anidulafungin, are the first-line treatment for candidemia or invasive candidiasis in critically ill patients. There are conflicting data on the pharmacokinetic properties of anidulafungin in intensive care unit (ICU) patients. Adult ICU patients (from 3 hospitals) receiving anidulafungin for suspected or proven fungal infections were included in the present study. Patients were considered evaluable if a pharmacokinetic curve for day 3 could be completed. Twenty-three of 36 patients (7 female and 16 male) were evaluable. The median (range) age and body weight were 66 (28 to 88) years and 76 (50 to 115) kg, respectively. Pharmacokinetic sampling on day 3 (n = 23) resulted in a median anidulafungin area under the concentration-time curve from 0 to 24 h (AUC0-24) of 72.1 (interquartile range [IQR], 61.3 to 94.0) mg·h·liter-1, a median daily trough concentration (C24) of 2.2 (IQR, 1.9 to 2.9) mg/liter, a median maximum concentration of drug in serum (Cmax) of 5.3 (IQR, 4.1 to 6.0) mg/liter, a median volume of distribution (V) of 46.0 (IQR, 32.2 to 60.2) liters, and a median clearance (CL) of 1.4 (IQR, 1.1 to 1.6) liters·h-1. Pharmacokinetic sampling on day 7 (n = 13) resulted in a median AUC0-24 of 82.7 (IQR, 73.0 to 129.5) mg·h·liter-1, a median minimum concentration of drug in serum (Cmin) of 2.8 (IQR, 2.2 to 4.2) mg/liter, a median Cmax of 5.9 (IQR, 4.6 to 8.0) mg/liter, a median V of 39.7 (IQR, 32.2 to 54.4) liters, and a median CL of 1.2 (IQR, 0.8 to 1.4) liters·h-1. The geometric mean ratio for the AUCday7/AUCday3 term was 1.13 (90% confidence interval [CI], 1.03 to 1.25). The exposure in the ICU patient population was in accordance with previous reports on anidulafungin pharmacokinetics in ICU patients but was lower than that for healthy volunteers or other patient populations. Larger cohorts of patients or pooled data analyses are necessary to retrieve relevant covariates. (This study has been registered at ClinicalTrials.gov under identifier NCT01438216.).