A radiopharmaceutical consisting of an estradiol analogue radiolabeled with the positron-emitting isotope fluorine F 18. F-18 16 alpha-fluoroestradiol is actively taken up in tumor cells expressing the estrogen receptor (ER), allowing visualization of ER-positive tumor cells with positron emiision tomography (PET). Uptake of this agent depends upon the ER status of target tissues.