ObjectiveTo describe the incidence of hypophosphataemia in patients admitted to the ICU who have required mechanical ventilation. To analyse the presence of risk factors and its relationship with nutritional practice. DesignProspective observational study. SettingPolyvalent ICUs of two University Hospitals. PatientsPatients on invasive mechanical ventilation > 72 h with normal level of phosphorus at admission. InterventionsNone. Main variables of interestElectrolyte levels (phosphorus, magnesium, and potassium) were determined on admission to the ICU and at 96 h. Risk categories on admission, caloric intake, insulin doses and acid–base status during the first 4 days of admission were recorded. Incidence was calculated as the percentage of patients who developed hypophosphataemia after admission. Univariate analysis was performed for between-group comparison and multivariate analysis of potential risk factors. ResultsEighty nine patients were included. The incidence of hypophosphataemia was 32.6%. In these patients phosphorus decreased from 3.57 ± 1.0 mmol/L to 1.87 ± 0.65 mmol/L (52.3%). The mean kcal/kg/24 h provided in the first 4 days was 17.4 ± 4.1, with no difference between the group that developed hypophosphataemia and the group that did not. Significant risk factors were insulin doses administered and pH and PaCO2 values. ConclusionsThe incidence of hypophosphataemia at 96 h from admission in mechanically ventilated patients is high and unrelated to the risk category and hypocaloric nutritional practice used. Insulin doses and acid–base status are the main determinants of its occurrence.