Ulnar nerve compression in Guyon's canal: MRI does not always have the answer

Lipoma is a rare cause of ulnar nerve compression in Guyon's canal. All four previously reported cases from 2000 to 2009 have been accurately diagnosed on MRI. We present a case report where the MRI and surgical findings differed and a summary of the previous cases in the literature. We conclude that although MRI remains the best investigation for this condition, it is not always accurate and clinical findings still provide the best basis for surgical treatment.