Background: Pituitary enlargement, which can regress with time, has been described in a number of PROP1-deficient patients. We report a PROP1-deficient patient with a unique variation in pituitary size. Case Description: A 4-year-old boy was first examined in 1989 for short stature (–2.3 standard deviation score). Growth hormone (GH) insufficiency was confirmed, and human GH (hGH) therapy was initiated and administered up to the age of 18.2 years. Levothyroxine was added 6 months after hGH initiation. Pituitary magnetic resonance imaging (MRI) obtained when the patient was 5 years old showed an enlarged pituitary gland, which grew larger by the age of 8.5 years and then regressed to normal size by the time the patient was 9.8 years old. MRI when the patient was 19 years old disclosed pituitary reenlargement, and another 3 years later indicated regression. On DNA analysis, the patient was found to be homozygous for the mutation 301–302ΔGA of the PROP1 gene. When the patient was 18.8 years old and asymptomatic, an impaired cortisol response to glucagon was detected. Conclusions: Regression of the pituitary enlargement in PROP1-deficient patients does not seem to constitute an end stage with respect to pituitary pathology. Further changes in pituitary morphology and size can be expected; therefore, long-term follow-up with pituitary MRI is advised.