A 3.5-year-old male Golden Retriever with lethargy, generalized lymphadenopathy, and hepatosplenomegaly was determined to have malignant histiocytosis, based on the morphologic appearance of neoplastic cells by light and electron microscopic examination, evidence of erythrophagocytosis, the presence of diffuse nonspecific esterase activity, and immunohistochemical demonstration of vimentin intermediate filaments. Because of the appearance of abundant iron stores in the bone marrow, serum was obtained for determination of iron, total iron binding capacity, and ferritin values. Serum ferritin concentrations were markedly increased. Measurement of serum ferritin concentrations may be useful in supporting a diagnosis of malignant histiocytosis, and the monitoring of serum ferritin concentrations may be useful for assessing treatment and regression of the disease.