We report a case of 79-year-old man, who was diagnosed to have transient ACTH deficiency associated with polymyalgia rheumatica (PMR). The patient presented with sudden onset bilateral shoulder pain, which was gradually aggravated. Plasma ACTH was undetectable, and both serum cortisol and urinary 17-OHCS were very low. Other pituitary hormones were normal, suggesting that hypothalamo-pituitary-adrenal (HPA) axis is selectively damaged. However, within several weeks, plasma ACTH returned to normal, and showed a normal increase response to corticotropin-releasing hormone stimulation test. These results indicated that ACTH deficiency was only transient. After hydrocortisone (10 mg/day) was administered, his symptoms became suddenly improved. Based on those results and clinical course, ie, elevated erythrocyte sedimentation rate, negative rheumatoid factor and the typical symptoms, which showed improvement to glulcocorticoid therapy, the final diagnosis was PMR, which was associated with transient ACTH deficiency. This is the first report of a case of PMR, in which the HPA axis was examined in its very acute phase. It was demonstrated that the case was associated with the transient adrenocortical hypofunction, which was recovered during a short time. It is therefore possible that PMR may show a different responsiveness of HPA axis depending on its phases.