Ovine pulmonary adenocarcinoma (OPA), caused by Jaagsiekte sheep retrovirus (JSRV), is a disease of increasing concern in the sheep industry. There is no commercial antemortem test for OPA; therefore, an early evaluation phase study was undertaken to examine the accuracy of transthoracic ultrasound examination using a 5-6.5 MHz sector ultrasound machine widely available in veterinary practice in the UK to diagnose OPA. Restraint, preparation and examination time was restricted to five minutes per sheep to represent the cost limitations of commercial sheep farming. One hundred sheep were examined. All 41 cases identified with suspect OPA lesions during transthoracic ultrasound examination had the diagnosis confirmed at postmortem examination, while sheep without ultrasonographic changes characteristic of OPA had no gross lesions of OPA at postmortem examination. This demonstrates the specificity of transthoracic ultrasound for diagnosis of OPA. The authors propose that, in the absence of any other reliable preclinical diagnostic test, the use of transthoracic ultrasound examination should be considered for a second opinion on an initial diagnosis of OPA, for screening purchased adult flock replacements for OPA, or for screening sheep in a known OPA-affected flock. However, the authors emphasise that a negative scan cannot provide a guarantee that the animal is free of JSRV infection nor early OPA.