Leuprorelin (leuprolide, D-Leu6-(des-Gly10-NH2)-LH-RH ethylamide) acetate is a super-active agonist of luteinizing hormone-releasing hormone (LH-RH). We developed once-a-month injectable microcapsules of this agonist by our novel in-water drying method. This depot formulation can release the drug at an apparent zero-order rate over one month with bioerosion of copoly (lactic/glycolic acid) utilized as a wall material of the polycore microcapsules. A dramatic prolonged depression of pituitary-gonadal axis, chemical castration, was achieved by the once-a-month injection in experimental animals; it expects a reliable efficacy for treating hormone-dependent prostatic, breast cancers and endometriosis. Studies on the dosage design of this new delivery system of leuprorelin are summarized.