Clinical study was conducted on 14 cases of large prolactinoma. Such cases were most common in patients in their 20s and 30s; i.e. in comparatively young persons. Of the total, 7 were male and 7 were female. Visual disturbance was observed in all cases (anisocoria 3, disturbance of EOM 4, cerebellar signs and hearing disturbance 1, amenorrhea 4, and primary amenorrhea 2). Two cases (female) were accompanied by galactorrhea. Prolactin in blood showed unusually high values in the range of 1, 050 to 15, 000 ng/ml. Incidences of impaired growth hormone response to ITT was 92%, that of impaired LH and FSH responses to LH-RH were 69 and 61 %, respectively, and that of impaired TSH response to TRH was 38%. Plain skull films revealed irregular destruction of sella turcica in many cases. Pneumoencephalography and CT scan showed 7 cases of suprasellar extension. In the remaining 7 cases, 2 cases of parasellar extension besides suprasellar extension, 2 temporal extension, one posterior (infratentorial) extension, and 2 panaivasive extension, with a tendency to further develop and extend in all directions, were noted. For these reasons, results of removal of the tumors by the transfrontal approach were insufficient in many cases. In one case, decline of prolactin in blood was insufficient even after extirpation, but reduction of the size of tumor and decrease of prolactin were obtained by the administration of bromocryptine (CB-154).