This study examined the determinants of compliance with clinical guidelines for glucocyte colony-stimulating factor (GCSF), a biotechnology product designed to reduce postchemotherapy infections. The pattern of compliance did change over time. After the guidelines were disseminated, appropriate use of GCSF increased. However, inappropriate use also increased. Patients who were younger and had an attending physician who was an oncologist or hematologist were more likely to receive GCSF whether they met the guideline criteria or not. Our findings suggest that older patients may be treated less aggressively than others and that physicians who are the most knowledgeable about guidelines may feel the most qualified to override the guidelines when they believe they do not apply. Our findings also demonstrate that it is easier to encourage physicians to do more for patients rather than less.