Serial serum ferritin (SF) levels were measured in 36 patients with neuroblastoma seen at Memorial Sloan-Kettering Cancer Center (MSKCC) between January 1981 and December 1982. The significance of the associations among SF, stage and extent of disease, number of blood transfusions, liver function, serum iron (Fe), total iron-binding capacity (TIBC), and transferrin saturation was investigated. Although a dominant statistical correlation was found between SF and number of blood transfusions, the results suggest that amount of disease contributes to increasing SF levels. Serum ferritin levels increased on average in a linear fashion with number of blood transfusions in patients free of disease or with minimal disease. In patients with bulky disease, this increase was exponential (p value less than 0.01). Application of a reverse hemolytic plaque assay to the analysis of ferritin secretion by cells demonstrates that tumor cells do secrete ferritin in vitro.