1) Nine of 25 cases of chromophobe adenoma revealed abnormally high plasma prolactin level. The plasma prolactin level of these 9 cases ranged from 5300 to 15000 ng/ml, and those of other 16 cases ranged from negligible to 53 ng/ml. 2) Five cases with abnormally high plasma prolactin level had a poor clinical course. 3) Quantitative determination on PEG according to our own method showed that tumor size of these patients with abnormally high plasma prolactin level was larger than that of relatively lower plasma prolactin level. 4) Three of all 25 cases of chromophobe adenoma were histologically diagnosed as pleomorphic. Each of those three cases showed abnormally high plasma prolactin level.