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In view of the diagnostic value of bilateral simultaneous inferior petrosal sinus sampling (BSIPSS) in localising preoperatively the site of the microadenoma in pituitary dependent Cushing's disease, the clinical value of BSIPSS was evaluated in five acromegalic patients with equivocal or negative pituitary CT scans. Intersinus GH gradients were obtained for all patients (range 1.6-4.2) but in only one case was the gradient correctly localised to the side of the tumour. Gradients of several other pituitary hormones, particularly prolactin (range 1.6-4.0), also demonstrated gradients parallel to the GH intersinus gradients. Despite the paradoxical intersinus GH gradients, the surgeon was able to readily identify the tumour at the time of surgery. Thus BSIPSS is of little assistance to the surgeon for the preoperative radiological evaluation in acromegaly and these results caution against the universal adoption of the BSIPSS technique in the radiological assessment of all secretory pituitary microadenomas.