Eighty-nine patients infected with Entamoeba histolytica, non-pathogenic amoebae or Trichuris trichiura were studied prospectively to determine the safety and efficacy of diphetarsone therapy. An additional 75 patients were studied retrospectively to assess further the efficacy of diphetarsone in the treatment of E. histolytica cyst passers. Side effects were noted in 9% and included gastrointestinal upset, lightheadedness and headache. Transient liver function abnormalities were recorded in 5.6%. diphetarsone was completely effective in the treatment of Dientamoeba fragilis, Entamoeba hartmanni, Iodamoeba buetschlii and Trichuris trichiura. 99% of the patients with E. histolytica, 97% of those with E. coli and 98% of those with Endolimax nana were cured.