The invention relates to the use of arsenic compounds to treat a variety of neoplastic diseases. The present invention encompasses the administration to a mammal of arsenic in the form of a salt, complex, organic compound or ionic solution to treat tumors of epithelial tissue, connective tissue, central nervous system, lymphoid tissue, hematopoietic cells and tumors associated with oncogenic viruses. This invention also encompasses the treatment of hematopoietic disorders in mammals by the administration of one or more arsenic compounds to said mammal. Further, the arsenic compounds may be used to treat metastatic neoplastic diseases.