Epidural myeloblastoma, which compressed the spinal cord, was the first evidence for chronic granulocytic leukemia, eosinophilic type, Ph chromosome negative. This manifestation was preceded by 3 years follow-up of a patient with persistent eosinophilia of 60% mature eosinophils. The only clues for the diagnosis of leukemia were splenomegaly and high serum vitamin B12, most of which was bound to transcobalamin I. The latter finding presents a useful diagnostic criterium in myeloproliferative disorders.