Cases of inaccessible mediastinal parthyroid adenomas are rare (1% of parathyroid tumours). Previously, when the localization had been confirmed by imaging techniques, we performed exploratory cervicotomy, followed by sternotomy after explorations had been completed. This procedure required two operations and sternotomy sometimes gave blank results. We therefore prospectively evaluated technitium-99m labelled methoxy-isobutyl isonitrile scintigraphy to localize an adenoma in the anterior mediastinum. A 39-year-old patient was hospitalized for primary hyperparathyroidism which had caused nephrocalcinosis and decalcification. Serum calcium was 130 g/L and phosphorus 16 mg/L. Cervical echography, thallium scintigraphy and computed tomography were negative. Scintigraphy with technitium-99m labelled methoxy-isobutyl isonitrile visualized a tumoural formation in the upper mediastinum near the ascending aorta. Magnetic resonance imaging centered on this zone revealed a 13 x 8 mm formation. Cervicosternotomy was performed initially. A 280 mg parathyroid adenoma was removed. The three other parathyroid glands were found to be normal. Follow-up was uneventful and the serum levels of calcium and phosphorus rapidly returned to normal. This preliminary case has demonstrated that methoxy-isobutyl-isonitrile scintigraphy can be an effective first intention exploration for primary hyperparathyroidism. Further results should indicate its sensitivity and specificity.