During the period 1985-1989 I observed 35 patients of both sexes and of all ages, who were suffering from signs and symptoms of clear blastocystosis, such as prevailingly watery diarrhea, abdominal pain, nausea, tenesmus, eosinophilia and fever. 5 of them showed concomitant diseases, like Acquired Immune Deficiency Syndrome (AIDS) or AIDS Related Complex (ARC), diabetes mellitus and nephrocarcinoma. By means of the administration of metronidazole 2 g/die for 5 days on average, I obtained the disappearance of Blastocystis hominis from the stools and the absence of the clinical symptomatology in 11 patients, while in another 7 the treatment did not prevent, in the following months, the arising of one or more clinical and microbiologic relapses.