Progesterone receptors (PRs) are present in many breast tumors, and their levels are increased by certain endocrine therapies. We describe the synthesis and PR binding affinities of a series of bromine-and iodine-substituted 16α, 17α-dioxolane progestins, some of which, when appropriately radiolabeled, are potential agents for diagnostic imaging of PR- positive breast tumors using positron emission tomography (PET) and for radiotherapy. ...