Sixteen patients were re-operated on for treatment of their hyper-parathyroidism. After analysis of the cause of failure or relapse, the authors discuss the examinations which led to a definite diagnosis and, in some cases, pre-operative localisation of the lesion. All the patients were treated by the cervical route only. Operation may be very difficult Technical advice is given here. Tetany and recurrent laryngeal nerve paralysis are common complications. They are often due to an inefficacious or incomplete first operation.