Although use of colony-stimulating factor (CSF) is widespread and guidelines for use have been disseminated, actual practice patterns of medical oncologists are unknown. The purpose of this study was to collect these data using an office-based computerized clinical information system. Data were collected on patients at 10 community-based oncology practices. Information regarding CSF use was captured at the time of prescribing through a computerized clinical support tool and stored in a data warehouse, and an analysis was carried out retrospectively. A total of 6,813 cancer regimens administered to 5,034 patients were evaluated for growth factor use. Overall, CSFs were used in 14% of regimens, with breast, lymphoma, lung, and ovarian being the most common cancers for which CSFs were used. In 49.4% of regimens, CSF was initiated during cycle 1, with an average duration of 1 week, and was used in two or three cycles per regimen. Afebrile neutropenia is rarely followed by CSF initiation. Granulocyte colony-stimulating factor (G-CSF) is associated with fewer dose adjustments, delays, and hospitalizations when compared with granulocyte-macrophage colony stimulating factor (GM-CSF). There is wide variation among oncologists in CSF use, and several substantial differences were noted between the prescribing behavior of American Society of Clinical Oncology (ASCO) survey-reported oncologists and actual clinical practice, as captured by the computerized clinical information system. Computerized clinical information systems can collect detailed information regarding practice patterns of medical oncologists. ASCO physician practice survey data do not accurately reflect actual practice patterns and must be interpreted with caution. Substantial deviations from ASCO growth factor guidelines remain, and oncologists' use of CSFs demonstrates wide variation. There may be important clinical differences between G-CSF and GM-CSF, but definitive phase III trials are needed for confirmation.