A pituitary mass compressing the optic nerve was revealed by magnetic resonance imaging (MRI) in a 35-year-old woman complaining of visual disturbance in the post-partum period. Responses of plasma gonadotropin and corticotropin-cortisol levels to respective hypothalamic hormones were delayed or blunted, but the response of plasma prolactin to thyrotropin-releasing hormone was exaggerated. Diabetes insipidus was not associated. Biopsy revealed lymphocytic adenohypophysitis, and no hypophysectomy was performed. Only five weeks later, the pituitary mass spontaneously disappeared on MRI. The pituitary function was normalized. Anti-thyroidal and anti-pituitary antibodies were negative throughout the clinical course. Pituitary masses developing during late pregnancy or the post-partum period should be carefully observed.