The use of adjuvant chemotherapy in women with node-negative breast cancer has been controversial and actively debated since the 1988 National Cancer Institute Clinical Alert. We developed a decision analysis model that used the results of available randomized controlled trials to assess the potential clinical and financial effects of using adjuvant chemotherapy for groups of 45-year-old and 60-year-old women. Using the baseline assumptions, we found that chemotherapy increases quality adjusted life expectancy and survival by a substantial amount at a cost comparable to most accepted medical interventions. The model highlights the uncertainties in duration of benefit from therapy, the need for refinements in risk stratification, the importance of patient preferences about toxicity and benefit, and the need for accurate cost-accounting for oncologic therapies. Decision analysis complements other methods for information gathering, analysis, and synthesis used in clinical research. With the increasing focus on the effectiveness of medical interventions, decision analysis will be an important tool for oncologists to understand.