Bone marrow mast cell content was evaluated by a semiquantitative method in 22 marrow specimens from 20 patients with preleukemic syndrome and was compared with 21 marrow specimens from iron-deficient control subjects. Results indicate a statistically significant increase of bone marrow mast cell content in patients with preleukemic syndrome in comparison to control subjects (p less than or equal to 0.0005). Two of 20 preleukemic patients converted to acute myeloblastic leukemia and conversion was accompanied by a significant decrease of bone marrow mast cell content. Our findings indicate that bone marrow mast cell content can be reproducibly quantitated and represents an additional morphologic criterion for diagnosis of the preleukemic syndrome.