The aim of this study was to assess the association of phosphate concentration with key clinical outcomes in a heterogeneous cohort of critically ill patients. Materials and Methods: This was a retrospective observational study at a general intensive care unit (ICU) of an Australian university teaching hospital enrolling 2730 adult critically ill patients. Results: We studied 10 504 phosphate measurements with a mean value of 1.17 mmol/L (measurements every 28.8 hours on average). Hyperphosphatemia (inorganic phosphate [iP] concentration > 1.4 mmol/L) occurred in 45 and hypophosphatemia (iP = 0.6 mmol/L) in 20 . Among patients without any episodes of hyperphosphatemia, patients with at least 1 episode of hypophosphatemia had a higher ICU mortality than those without hypophosphatemia (P = .004). In addition, ICU nonsurvivors had lower minimum phosphate concentrations than did survivors (P = .009). Similar results were seen for hospital mortality. However, on multivariable logistic regression analysis, hypophosphatemia was not independently associated with ICU mortality (adjusted odds ratio, 0.86 [95 confidence interval, 0.66-1.10]; P = .24) and hospital mortality (odds ratio, 0.89 [0.73-1.07]; P = .21). Even when different cutoff points were used for hypophosphatemia (iP = 0.5, 0.4, 0.3, or 0.2 mmol/L), hypophosphatemia was not an independent risk factor for ICU and hospital morality. In addition, timing of onset and duration of hypophosphatemia were not independent risk factor for ICU and hospital mortality. Conclusions: Hypophosphatemia behaves like a general marker of illness severity and not as an independent predictor of ICU or in-hospital mortality in critically ill patients.