The latest developments and ideas in the therapy of leprosy are discussed, the need for long-term studies being stressed. The therapeutic efficacy and effective dosages of some sulfones (especially diaphenylsulfone), thiambutosine and long-acting sulfonamides such as sulfamethoxine and sulfalene, are considered. The possibilities for two newer drugs, 4,4'-diacetyldiaminodiphenylsulfone and clofazimine (B-663), both still in the early stages of evaluation, are also described and the potential value of thalidomide in treatment of the lepra reaction is discussed. The authors make a number of recommendations for controlled trials and lines of investigation and, in particular, favour a biochemical approach to the correction of defective host defences. Diaphenylsulfone is still considered the drug of choice for use in the therapy of leprosy.