Abstract: Iodotamoxifen derivatives which are compounds of formula (3) ##STR1## wherein X represents 3- or 4- iodo and R.sup.1 and R.sup.2, which may be the same or different, represent C.sub.1-3 alkyl, especially methyl or ethyl, groups or R.sup.1 represents a hydrogen atom and R.sup.2 a C.sub.1-3 alkyl group or R.sup.1 and R.sup.2 together with the nitrogen atom to which they are attached represent a saturated heterocyclic group, especially a pyrrolidino, piperidino, 4-methylpiperidino or morpholino group, and their pharmaceutically acceptable acid addition salts are potent anti-estrogenic compounds useful for treatment of estrogen receptor-positive (hormone-dependent) breast cancers.Radioisotopic iodotamoxifen derivatives of formula (3) are useful in radiotherapy or gamma ray imaging of these cancers.