A 56‐year‐old woman presented with diabetes mellitus and primary hyperparathyroidism simultaneously. Initial random blood glucose was recorded at 20.8 mmol l−1, serum calcium was 3.07 mmol l−1 (normal range 2.10–2.55 mmol l−1), and plasma parathyroid hormone estimation by intact assay was 110 ng l−1 (normal range 10–65 ng l−1). Initial glycated haemoglobin was 9.4 % (non‐diabetic range < 7.5 %). Left lower parathyroidectomy was carried out and pathology confirmed the presence of a chief cell adenoma. The gland measured 10 × 5 × 5 mm. Following parathyroidectomy serum calcium normalized, glucose tolerance improved, and a subsequent 75 g oral glucose tolerance test was normal. The patient weighed 80 kg at presentation but the post‐operative weight had risen to 81.5 kg. The most recent glycated haemoglobin was 4.6 %. Primary hyperparathyroidism may have a reversible effect on glucose tolerance. © 1997 by John Wiley & Sons, Ltd.