Abstract Bragg-peak proton hypophysectomy was used in 22 acromegalic patients. Of 14 followed for two to 36 months, size of hands, feet or face decreased in nine. Variable improvement took place in headache, fatigability, heat intolerance, joint pain, sexual function, weight, skin, tachycardia, hypertension, visual fields, glucose tolerance, thyroid-function tests and ketosteroid and hydroxysteroid secretions. Levels of human growth hormone fell strikingly in eight and to an insignificant degree in four, and increased in two. These effects were ordinarily progressive over the follow-up months, and longer observations are necessary. There has been no mortality, cerebrospinal-fluid leak, infection or diabetes insipidus. Complications included transient headache, occasional diplopia and one case of anterior pituitary insufficiency. The first patient treated had serious unilateral visual loss. The method compares favorably with other procedures in its ability to suppress human growth hormone and in the infreq...