No adverse allergic reactions to iodine-131-metaiodobenzylguanidine ((131)I MIBG) at a diagnostic dose have been reported in the English literature. This report of a skin eruption in a 35-year-old man after an intravenous injection of (131)I MIBG strongly suggests an adverse allergic reaction, and is the first to address such a side effect of (131)I MIBG at a diagnostic dose. Erythematous maculopapular eruptions, some of which were contiguous, were seen in a symmetric disposition on the patient's chest walls, elbows, neck and face 18 h after the (131)I MIBG injection. Antiallergic treatment resolved the lesions completely. There were no possible causes of the exanthema other than the (131)I MIBG injection. Urticaria related to the (131)I MIBG injection and caused by type I allergic reaction was suspected, and these findings point to the possible risk of a hitherto unreported allergic skin reaction to (131)I MIBG. We would like to draw the attention of nuclear physicians to this possible drawback of (131)I MIBG.