This prospective study examined the accuracy of ultrasound in diagnosing occult groin hernias in adults. The study included 52 consecutive patients reviewed in the surgical out-patient clinic with a history suggestive of groin hernia but with a normal or inconclusive clinical examination. Each patient underwent a preliminary ultrasound examination by an experienced consultant radiologist who was aware that the patient had a history suggestive of a hernia but was blinded to the side of the symptoms. The patient then proceeded to herniography, and some patients also had surgical exploration. The results of the ultrasound were assessed in relation to the herniography, and the patients who proceeded to surgical exploration had further correlation with surgery. Ultrasound had a sensitivity of 29% and specificity of 90% compared with the herniography. Correlation with surgical findings showed ultrasound to have a sensitivity of 33% and a specificity of 100%. The sensitivity of ultrasound in detecting clinically occult hernias in a non-acute presentation is poor, and patients with normal ultrasound should be considered for further investigation.