A 17-year-old boy with hyperparathyroidism exhibited a large cyst in the left hemi-thorax on his chest X-ray. The cyst was identified as a hydatid cyst. He was referred for biphasic methoxy isobutyl isonitrile (MIBI) parathyroid scan. A discrete ectopic parathyroid was detected in the left upper mediastinum in addition to accumulation of the MIBI tracer in the cyst wall. Using radio-guided occult lesion localization with an injection of 3 mCi 99m Tc - MIBI in the morning of the operation day, an ectopic adenoma was detected with a gamma probe within the thymus, resected with a consequent decline of serum high parathyroid hormone (PTH). This report is remarkable in two different aspects: the importance of radio-guided localization of ectopic adenoma, which in this case was within the thymus, and the accumulation of MIBI in hydatid cyst wall, as a unique report of this condition.