The study of the cerebrospinal fluid (CSF) abnormalities in 55 patients with acute anterior poliomyelitis is made. The lumbar puncture was performed in the preparalytic stage or, at least, in the first week after onset of paralysis. The findings were much alike that one of the literature, but the authors emphasize some peculiarities. All 55 samples of CSF showed some abnormalities, generally pleocitosis of lymphocytic type and moderately increased protein content. The fibrin clot was a very common finding (60%), and in many cases (22%) it appeared in absence of increased protein content. In a case the only abnormality was a delicate fibrin clot. In two cases the only abnormality found in CSF was: in one, neutrophil granulocyte and in the other, eosinophil granulocyte in spite of normal cell count.