Purpose: The authors investigated the effects of a novel antipsychotic agent, quetiapine administration on hypothalamo-pituitary-gonadal (HPG) axis hormones in patients with chronic schizophrenia. Methods: In this prospective, open-label study, the subjects were nine male inpatients. We tried to switch the patients from typical antipsychotics to quetiapine. The patients took a mean dose of 636.1 mg quetiapine daily for a mean period of 104.7 days. Results: (i) Although the total Brief Psychiatric Rating Scale score did not show significant changes, no patients needed anticholinergic medications after the switch. (ii) Before the switch, only the prolactin concentration showed a significant difference between the patients and normal subjects. After the switch, neither prolactin nor HPG axis hormone concentrations showed significant differences between the groups. Conclusions: Our preliminary results indicated that quetiapine might not affect prolactin and HPG axis hormones at least in chronic schizophrenia patients with normal levels of HPG axis hormones.