Background: Patients admitted to the Intensive Care Unit (ICU) often have low magnesium, phosphate and zinc levels. Monitoring of serum concentrations and supplementation may be important, but there is no consensus on optimal practice. The objective of the WhyTrace survey was to describe current practice regarding the measurement and supplementation of magnesium, phosphate and zinc in ICUs. Methods: A 54-item electronic questionnaire was developed in accordance with SURGE, SUrvey Reporting GuidelinE, to address international clinical practice in the ICU. National investigators recruited ICUs in ten countries with one physician responding per ICU using a unique e-mail distributed survey-link. Results: The questionnaire was sent to clinicians in 336 ICUs of whom 283 (84%) responded. In 62% of the ICUs, a standard procedure was in place regarding the measurement of serum magnesium levels, in 58% for phosphate and in 9% for zinc. Zinc was never or rarely measured in 64% of ICUs. The frequency of requesting serum levels varied from twice daily to once weekly. Regarding supplementation, 66% of ICUs had a standard procedure for magnesium, 63% for phosphate and 15% for zinc. Most procedures recommended supplementation when serum levels were below the lower reference level, but some used the upper reference levels as the threshold for supplementation and others decided on a case-by-case basis. Conclusion: The practice of measuring and supplementing magnesium, phosphate and zinc differed substantially between ICUs. Our findings indicate that there is a need for high-quality prospective data on frequencies of measurements, treatment goals and effects of supplementation on patient-important outcomes.