Clozapine (Clozaril, Novartis) is classified as a prolactin-sparing antipsychotic, so clozapine-induced hyperprolactinemia is rare. Herein, however, we present a case of severe hyperprolactinemia with delusion of pregnancy during the use of clozapine. We discuss various factors involved in the background of hyperprolactinemia and delusion of pregnancy. While clozapine-induced hyperprolactinemia is rare, the adverse events of this condition can be clinically important in the short- and long-term. In Japan, the measurement of prolactin levels is not a required item in the Clozaril Patient Monitoring Service (CPMS, Novartis), so the risk of hyperprolactinemia tends to be easily overlooked. We hope this case report will help to alert colleagues to the risk of hyperprolactinemia during clozapine use.