The appropriate justification for using a diagnostic or therapeutic intervention is that it provides benefit to patients, society, or both. For decades, indwelling arterial catheters have been used very commonly in patients in the ICU, despite a complete absence of data addressing whether they confer any such benefits. Both of the main uses of arterial catheters, BP monitoring and blood sampling for laboratory testing, can be done without these invasive devices. Prominent among complications of arterial catheters are bloodstream infections and arterial thrombosis. To my knowledge, only a single observational study has assessed a patient-centered outcome related to arterial catheter use, and it found no evidence that they reduce hospital mortality in any patient subgroup. Given the potential dangers, widespread use, and uncertainty about consequences of arterial catheter use in ICUs, equipoise exists and randomized trials are needed. Multiple studies in different, well-characterized, patient subgroups are needed to clarify whether arterial catheters influence outcomes. These studies should assess the range of relevant outcomes, including mortality, medical resource use, patient comfort, complications, and costs.