Out of three cases of neuroblastoma under observation, the diagnosis could be established from the cytological and electron microscopical examinations of bone marrow in two of them. Slide smears revealed the rosette-shaped pattern characteristic of malignant neuroblastoma, many of which were fitted with dendritic plasmatic processes. Electronography helped to prove intraplasmatically the presence in those cells of dense neurosecretory granules, and--in their plasmatic processes--the presence of microfilaments, microtubules, neurosecretory granules and empty vesicules.