Twenty-three patients with primary hyperparathyroidism were examined by MR, CT and sonography in order to localise the parathyroid adenoma. In twenty patients, surgery was performed and the findings confirmed histologically. Fifteen patients had an adenoma in a parathyroid and in three, the adenoma was in the mediastinum. In one patient there was hyperplasia of all parathyroids. Accuracy of MR was 98%, of CT 98% and sonography 94% in previously unoperated patients and is therefore similar for these methods. This is also true for previously operated patients (four) where MR was 88% accurate, CT 83% and sonography 91%. In sixteen patients without previous operation, MR had the highest sensitivity with 79%. In previously operated patients, the sensitivity of MR and sonography was equal, with 67%, and CT was similar with 66%, MR was able to differentiate adenomas by the specific measurements of T1 and T2.