Gonadotropin-releasing hormone (GnRH) agonists are used for gonadal suppression in the treatment of breast and prostate cancers. In older men, their use has occasionally been associated with cardiovascular side effects such as supraventricular tachyarrhythmias (SVTs). Several reports document their occurrence in men receiving leuprolide for prostate cancer. We now report this complication with concomitant occurrence of migratory trunk and extremity urticaria in a young woman receiving this treatment after diagnosis of a T1cN0 premenopausal breast cancer. Changing from leuprolide to another GnRH agonist, goserelin, no additional problems with SVT or accompanying urticaria were encountered during the nearly two years of treatment and three subsequent years of follow-up.