A method for treating a lymphoma selected from non-Hodgkin's and Hodgkin's lymphoma comprising administering an organoarsenic compound having a structure of the formula (I) wherein X is S or Se and R1 and R2 are independently C1-30alkyl (R3, R3′, R4, R5, W and “n” are as defined in claim 1) in particular where the compound is S-dimethylarsinoglutathione, N-(2-S-dimethylarsinothiopropionyl)glycine, 2-amino-3-(dimethylarsino)thio-3-methylbutanoic acid, S-dimethylarsino-thiosuccinic acid or S-dipropylarsino-1-thioglycerol.