OBJECTIVE The immediate early gene locus AP-1, incorporating the cellular oncogenes c-fos and c-jun (and their oncoprotein products Fos and Jun respectively) play a key role in regulating cell growth and differentiation. The myc-gene is also known to promote cell growth. In order to investigate the possible role of these oncogenes in human pituitary adenomas, Fos, Jun and Myc oncoprotein immunoreactivities were assessed in surgically resected pituitary adenomas in relation to in-vivo characteristics (hormone secretion, size and invasiveness) and an in-vitro index of cell proliferation (Ki-67 immunoreactivity). Thirty-three human pituitary adenomas and 16 normal pituitary glands were examined. METHODS Oncoprotein immunoreactivity was recorded as present (+) or absent (-), and Ki-67 labelling indices were scored quantitatively. Tumour size was scored from CT scan appearances and radiographic evidence of bone erosion was noted. RESULTS Oncoprotein immunoreactivity was present in a total of 32/33 cases. Myc immunoreactivity was restricted to the only ACTH-secreting tumour in the series (1/33). Ki-67 immunoreactivity was present in 24/32 cases and labelling indices varied from 0.1 to 3.2%. CONCLUSIONS Oncoprotein immunoreactivity did not correlate with hormonal profile, bone erosion or the size of the proliferating compartment estimated by Ki-67 labelling indices. Although oncoprotein expression is common in human pituitary adenomas, its significance remains to be elucidated.