The purpose of this study was to describe the range of side effects experienced by a sample of patients receiving six cycles of cyclophosphamide, methotrexate, 5-fluorouracil (CMF) chemotherapy in the treatment of breast cancer. The study aimed to document the incidence and severity of side effects after each cycle of treatment and to determine which side effects patients perceived as most "troublesome." A sample of 52 women receiving chemotherapy on an outpatient basis completed a reliable and valid self-report questionnaire after each treatment cycle. The questionnaire listed 61 potential problems and prompted respondents to note any unlisted problems that they had experienced. Respondents assessed the severity of each problem on a five-point adjectival scale. The participants provided a total of 260 questionnaires assessable for data analysis. A total of 94 different side effects were reported. The most commonly reported problem was alopecia (mean incidence = 91%), followed by fatigue (89%), and weight gain (68%). Postchemotherapy nausea ranked 12th in terms of incidence. However, in response to an open question, patients judged fatigue and nausea to be the "most troublesome" problems, followed by difficulty sleeping and sore eyes. A most important implication of these findings is that the incidence of a side effect is not a reliable guide to the significance of the problem in terms of patient well-being; alopecia may be the most common problem identified, but it does not appear to be the problem that is most troublesome to patients. To further understand and alleviate the side effects of chemotherapy, the following measures are necessary. Patient education should be based on detailed, regimen-specific information. Routine, systematic assessment of the individual patient's side effects is desirable. Because many side effects appear to be controlled only moderately, healthcare providers must work toward more sophisticated, holistic symptom management strategies.