A case of primary hyperparathyroidism in a patient with myotonic dystrophy is reported. A 56-year old female with myotonic dystrophy, admitted to hospital with a urinary tract infection, had widespread muscle atrophy and myotonia with bilateral cataracts. Biochemical findings of normal renal function but raised blood calcium, depressed blood phosphate and increased parathyroid hormone, were consistent with a diagnosis of primary hyperparathyroidism. Thallium scanning of the parathyroids showed an area of discordant thallium suggesting a parathyroid adenoma. When the left lower parathyroid was later excised, histology was consistent with the diagnosis of parathyroid adenoma. As far as the authors are aware this is the first report of myotonic dystrophy and primary hyperparathyroidism in the same patient.