This study was aimed at detecting the early appearance of myelofibrosis by bone marrow biopsy examination in children with vitamin D-deficiency rickets. Twelve children, aged 4 to 18 months, were evaluated. Only a minimal increase of the reticulin was shown in rachitic children without anemia in whom no other laboratory evidence of myelofibrosis was present. Early signs of myelofibrosis with increase of reticulin were present in rachitic infants with anemia. In patients of the same age with iron deficiency anemia, the bone marrow reticulin findings were normal. Bone marrow biopsy after successful treatment of rickets could be repeated in one patient with myelofibrosis; results indicated that the myelofibrosis with anemia associated with vitamin D-deficiency rickets is reversible by vitamin D treatment.