Objective: To determine the biochemical effects of restricting the use of chloride-rich intravenous fluids in critically ill patients. Design: Prospective, open-label, before-and-after study. Setting: University-affiliated intensive care unit. Patients: A cohort of 828 consecutive patients admitted over 6 months from February 2008 and cohort of 816 consecutive patients admitted over 6 months from February 2009. Interventions: We collected biochemical and fluid use data during standard practice without clinician awareness. After a 6-month period of education and preparation, we restricted the use of chloride-rich fluids (0.9 saline, Gelofusine, and Albumex 4) in the intensive care unit and made them available only on specific intensive care unit specialist prescription.