Dissemination of use of the hematopoietic colony-stimulating factors (CSFs) is unprecedented in oncology, with almost all physicians having experience with granulocyte colony-stimulating factor (G-CSF) or granulocyte-macrophage colony-stimulating factor (GM-CSF) shortly after the drugs received Food and Drug Administration (FDA) approval in 1991. The American Society of Clinical Oncology (ASCO) Health Services Research Committee sought to assess patterns of use of CSFs before dissemination of its first-ever publication of ASCO guidelines. A questionnaire describing clinical scenarios was mailed to American oncologists and hematologists who practice medical oncology. In each scenario, the physician was asked whether he would prefer to use a CSF to prevent or treat neutropenia. The response rate to the mailed survey was 49% (N = 475). Most physicians preferred to use CSFs for secondary prophylaxis in patients receiving chemotherapy at rates of 44% to 85%, rather than reduce doses. Patterns of use did not differ for palliative, curative, or adjuvant chemotherapy. While the majority of CSF patterns of care were similar to those recommended in the ASCO guidelines, more than half of the physicians chose to use CSFs in the treatment of febrile neutropenia, an area not supported in the subsequent guidelines. In general, physicians at academic medical centers and in Health Maintenance Organization (HMO) practices were more likely to prefer dose-reduction strategies over addition of CSFs, while fee-for-service physicians preferred the opposite strategies. Variations in CSF preferences for use were related to differences in clinical characteristics (history of afebrile v febrile neutropenia), drug characteristics (G-CSF or GM-CSF), and physician practice characteristics (HMO or fee-for-service setting). However, before dissemination of the guidelines, the majority of American oncologists preferred strategies that were subsequently included in the ASCO CSF guidelines. CSF guidelines would be most likely to reduce CSF use for treatment of afebrile and uncomplicated febrile neutropenia.