We immunohistochemically examined the expression of leptin in pituitary adenomas to investigate the correlation between the invasiveness of tumours and leptin expression. The subjects consisted of 79 patients with pituitary adenoma and were classified into the following groups: (1) non-functioning adenomas; (2) GH-secreting adenomas; (3) prolactinomas; (4) ACTH-secreting adenomas; (5) others (LH, FSH or TSH-secreting adenomas). Thereafter all cases were subdivided according to the size of tumour and the presence of invasion to the surrounding tissue. Among non-functioning adenomas, there was no significant difference between invasive and non-invasive non-functioning adenoma. In functioning adenomas, a significant difference in leptin expression was noted in intrasellar non-invasive adenomas compared to other adenomas. There was also a significant difference in leptin expression between non-invasive and invasive adenomas regardless of size. These results suggest that leptin has a role in the invasive potential of functioning adenomas.