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Parathyroid tissue is sometimes auto-transplanted into the forearm after 'total' parathyroidectomy in patients with renal failure. Recurrent hyperparathyroidism demands identification of the source of PTH secretion which cannot be assumed to be the forearm. To this end, Tc-99m methoxy isobutyl isonitrile (MIBI) scintigraphy was used to identify functioning auto-transplanted parathyroid tissue in seven patients undergoing renal replacement therapy (five with functioning renal transplants and two on haemodialysis). Serum PTH was estimated in venous blood taken proximal and distal to the forearm graft and from the contralateral arm, and subsequent Tc-99m MIBI scanning was carried out without knowledge of the PTH results. Five patients had a significant gradient in PTH levels between sites proximal and distal to the graft, and between the proximal site and the contralateral arm, suggesting functioning parathyroid tissue in the graft. Subsequent Tc-99m MIBI scintigraphy confirmed the activity of the auto-transplanted parathyroid tissue in these five patients. In the remaining two patients without a significant PTH gradient between the sampling sites, Tc-99m MIBI scintigraphy did not identify any functioning forearm parathyroid tissue. The scan results therefore correlated well with the gradients in PTH levels, suggesting that MIBI scintigraphy can be used to identify functioning auto-transplanted parathyroid tissue. The results also indicate that any patient who has undergone auto-transplantation of parathyroid tissue must have blood samples taken from veins proximal to the graft and either distal to it, or from the contralateral arm when parathyroid status is re-assessed, particularly when surgery is being considered for recurrent hyperparathyroidism.