The difficult and disappointing cases encountered in patients who were operated upon for hyperparathyroidism between 1960 and 1987 have been studied. Ninety two patients underwent negative cervicotomy. Among the 12 patients who underwent a second operation, 4 were found to have cervical adenomas and in 6 others mediastinal adenomas were discovered by sternotomy. Most of the other cases were diagnostic errors. The present frequency of negative surgical operations is very low: 2 to 3 per cent of the cases; errors of diagnosis are exceptional, and the unusual sites of adenomas are better known. Seventeen patients were reoperated upon for persistent or recurrent hypercalcaemia despite excision of a 1st parathyroid lesion; 3 cancers which had been mistaken for adenomas at the 1st operation; 6 had a 2nd adenoma which had passed unnoticed during an exploration that was probably too limited and as hypercalcaemia persisted all 6 were reoperated upon with success at different intervals; finally, 8 patients had undiagnosed primary hyperplasia (as part of polyadenomatosis in 5 cases). Recurrences were observed, often after a long period, up to 3 to 5 years. Hypercalcaemia was corrected by the 2nd operation in only 5 out of 8 cases. In retrospect, it was found that the pathological examination could not always determine if the lesion was malignant or distinguish between adenoma and glandular hyperplasia.