Pharmacokinetic studies of daptomycin in septic patients indicate that pharmacokinetic parameters may be altered. The purpose of this clinical investigation is to determine the pharmacokinetics of daptomycin in a population of hospitalized patients with clinically significant gram-positive infections and receiving daptomycin. Daptomycin was measured using an isocratic HPLC technique. Thirty-five patients suffering from gram-positive severe infections and receiving daptomycin were included in the study. Patients were divided into two groups, depending on the dose of daptomycin received: group A, including 24 patients receiving 6 mg/kg/day daptomycin and group B, 11 patients receiving 8 mg/kg/day. Patients receiving a daptomycin dosage of 8 mg/kg/day had significantly higher values of mean C (max) and AUC(0-24). Each group was further divided into three subgroups, according to the creatinine clearance (CrCl) values: (1) patients with a CrCl >80 ml/min, (2) patients with CrCl ranging between 80 and 40 ml/min, and (3) patients with CrCl <40 ml/min. Compared to patients with normal renal function, those with CrCl <40 ml/min had higher mean values of minimum concentration (C (min)) (p < 0.001), AUC(0-24) (p = 0.03), and prolonged plasma half-time (p < 0.001). These differences were present both in patients receiving 6 and those with 8 mg/kg/day. However, in each of the three subgroups with different degrees of renal function a marked variability of pharmacokinetics parameters was observed. The factors associated with increased mortality were an infection acquired in the ICU, hypoalbuminemia, and AUC/MIC <666. The marked variability that characterizes daptomycin pharmacokinetics in these patients suggest the monitoring of the main pharmacokinetic parameters in this clinical setting.