Limited information about fatigue patterns in patients with cancer exists in the biotherapy literature. When fatigue is mentioned, it is usually to state whether or not it was a dose-limiting side effect. No further data are provided on how fatigue was measured; which patterns were noted and when; and which relationships were found between fatigue and demographic characteristics, type of biologic response modifier, route of administration, or cumulative dose. Thus, there is little available in the biotherapy literature to guide nursing practice in managing this side effect. Theory that guides practice, however, often emanates from the personal experiences of the patients and from the clinical observations and intuitive hunches of the nurses and physicians participating in clinical trials. These individuals have been most generous in sharing their insights and unpublished data with the authors. This paper presents a comprehensive view of current knowledge on fatigue to guide present nursing practice with patients receiving biotherapy and to provide direction for future nursing and clinical trial research.