Electron-microscopic examination revealed the presence of annulate lamellae in four of five prolactinomas. All the patients examined were 24--30-year-old women who had suffered from amenorrhea and galactorrhea for 3-5 years. Serum levels of prolactin were 114--4,700 ng/ml. The tumors removed by a transsphenoidal approach were 4-23 mm in diameter and showed the ultrastructural features of sparsely granulated prolactinoma. Analysis in correlation with clinical and laboratory data revealed that the annulate lamellae were found in rapidly growing prolactinomas. This is the fifth report on pituitary adenomas with annulate lamellae.