Potassium iodide was discovered as part of a war effort at the beginning of the 19th century in France. For the manufacture of gunpowder, sodium carbonate was extracted from seaweed ash. During this process, Courtois, a French chemist, discovered iodide in 1811. Potassium iodide was first used as a medicament for the therapy of goiter, which had been treated with seaweed ash. The therapeutic effect was excellent and dramatic. Thereafter, potassium iodide has been tried for a variety of diseases. In dermatology, it is now being used for the treatment of cutaneous deep mycosis and erythematous dermatoses including erythema nodosum, subacute nodular migratory panniculitis, erythema induratum, Sweet’s syndrome, and erythema exsudativum multiforme. The present author has found that potassium iodide works well as a first choice of medication for erythematous disorders. The action mechanism of the drug is still not fully understood.