A 36 y/o female presented with the chief complaint of diarrhea and vomiting which had lasted for four days, and with a family history of suicide. The first general examination showed severe dehydration with hyponatremia. After admission, she was diagnosed as having isolated adrenocorticotropic hormone (ACTH) deficiency and mixed connective tissue disease, and the steroid replacement therapy was started with the dose equivalent to 7.5 mg/day of prednisolone (PSL). Three days later, she had right sensorineural hearing loss (SNHL). She was given 40 mg/day PSL in addition to the steroid replacement therapy. On the next day, she developed a persecutory type of paranoid disorder, and then was given psychiatric medication. After tapering off PSL for SNHL, the delusion began to improve with psychiatric medication. Three weeks after the onset of SNHL, her hearing level had partially recovered. Ten months later, she did not show any psychic instability. A family history of psychosis and the present history of malnutrition and connective tissue disease are risk factors of steroid psychosis. It can develop even with 5 mg PSL if the patient has a risk factor. Careful medical history taking and knowledge about the steroid psychosis will prevent the severe side effects associated with steroid treatment.