An 18-year-old boy with delayed puberty was found to have a large prolactinoma. Hypophysectomy led to a fall in serum prolactin concentration, although it remained markedly elevated. The introduction of replacement therapy, including testosterone, resulted in painful swelling of the breasts and galactorrhoea. Bromocriptine therapy resulted in a cessation of galactorrhoea and normalization of serum prolactin concentrations. The absence of galactorrhoea in association with an astronomical concentration of prolactin, and its onset in association with diminished prolactin concentrations following hypophysectomy and the introduction of testosterone therapy, indicate that galactorrhoea requires the presence of high prolactin concentrations to act on mature breast tissue. This would account for the rarity of galactorrhoea in prepubertal children with prolactinomas.