Augmented renal clearance (ARC) is being increasingly described in neurocritical care practice. The mechanisms driving this phenomenon are largely unknown. The aim of this project was therefore to explore changes in renal function, cardiac output (CO), and atrial natriuretic peptide (ANP) concentrations in patients with isolated traumatic brain injury (TBI). This prospective observational cohort study was conducted in a tertiary level university affiliated ICU. Patients with normal plasma creatinine concentrations (< 120 μmol/L) at admission and no history of chronic kidney disease, admitted with isolated TBI, were eligible for enrolment. Continuous CO measures were obtained using arterial pulse waveform analysis. 8-h urinary creatinine clearances (CLCR) were used to quantify renal function. ANP concentrations in plasma were measured on alternate days. Data were collected from study enrolment until ICU discharge, death, or day 15, which ever came first. Eleven patients, contributing 100 ICU-days of physiological data were enrolled into the study. Most participants were young men, requiring mechanical ventilation. The median ICU length of stay was 9.6 [7.8-13.0] days. Elevated CLCR measures (> 150mL/min) were frequent, and appeared to parallel changes in CO. Plasma ANP concentrations were also significantly elevated over the study period (minimum value = 243 pg/mL). These data suggest that ARC is likely to complicate the care of TBI patients with normal plasma creatinine concentrations, and may be driven by associated cardiovascular changes, and/or elevated plasma ANP concentrations. However, significant additional research is required to further understand these findings.