In a two-centre study, intrasellar pituitary tissue pressure was measured at transsphenoidal surgery in 107 patients with pituitary adenomas or intrasellar cysts. 'Normal' ISP in patients under anaesthesia with small microadenomas (< 5 mm diameter) or a partial empty sella was 12 +/- 3 mmHg. Raised ISP (> 15 mmHg) was found in 75% of cases. The highest pressures were recorded in tumours with parasellar invasion (30 +/- 2; p < 0.01) irrespective of size and extension in other directions. In non-invasive lesions there was no correlation between the level of raised ISP and tumour size. Hypopituitarism and stalk compression syndrome were both associated with higher ISP than patients with normal pituitary function.