Estrones have been subjected to an addition reaction with [(4-ethynylphenyl)azo]pyrrolidine to provide 17 α-{4-[(pyrrolidin-1-yl)-azo]phenylethynyl}estra-3,17 β -diols. A subsequent iodination with iodotrimethylsilane, produced in situ from chlorotrimethylsilane and NaI, leads to 17α-(piodophenylethynyl) estra-3,17 β -diols. This reaction can also be carried out with Na¹²⁵ I to give radiolabelled estra-3,17 β -diols, which are potentially useful radiodiagnostic agents for the detection of estrogen positive breast cancer. The stability of the radiolabelled compounds is exemplified in a stability study of 3-O-methyl 17 α-(p-[¹²⁵ I]iodophenylethynyl)estra-1,3,5(10),6-tetraene-17 β -ol in acetonitrile.