The S810L mutation within the human mineralocorticoid receptor (MR S810L) induces severe hypertension and switches progesterone from antagonist to agonist. Here we report the crystal structures of the ligand-binding domain of MR S810L in complex with progesterone and deoxycorticosterone, an agonist of both wild-type and mutant MRs. These structures, the first for MR, identify the specific contacts created by Leu810 and clarify the mechanism of activation of MR S810L.