A prospective, parallel-group, randomized controlled trial was conducted in an emergency department. Patients being admitted to the ED or already admitted to the ED were included in the study if the physician attending the patient ordered an ABGA. Exclusion criteria were permanent mental disability, patient age < 18 years, patients declining to participate in the study or ABGA contraindicated. Patients were randomly assigned to arterial puncture using the standard procedure, or to US guided puncture. The primary endpoint of the study was the proportion of patients in which arterial puncture for ABGA was successful in the first attempt.