This study examines the role of bilateral synchronous inferior petrosal sinus sampling (BSIPSS) in resolving two major issues in the pre-operative assessment of Cushing's disease, namely proof of pituitary dependent disease and accurate lateralisation of tumour within the pituitary. BSIPSS was technically successful in 16 of 20 patients. The central to peripheral ACTH gradients, supporting the diagnosis of pituitary dependent disease, was greater than 2.0 (2.0-27.2) in all patients with histologically proven ACTH-secreting pituitary tumours and in those who remained in remission following pituitary surgery. In addition, BSIPSS accurately localised the site of the tumour within the pituitary in 13 of the 16 technically satisfactory studies and thus contributed to the outcome of surgical treatment. In contrast CT scan demonstrated a definite tumour in only two patients.