We have analyzed the relationship between dose intensity of cyclophosphamide, methotrexate, and fluorouracil (CMF)-containing adjuvant chemotherapy of stage II breast cancer and 3-year relapse-free survival. Studies using only one or two drugs of CMF or melphalan instead of cyclophosphamide were included in the analysis by using simple techniques developed for this purpose. There was a clear-cut relationship between relapse-free survival and dose intensity in trials containing all four prognostic groups: less than 50 years, one to three and more than three positive nodes; and greater than or equal to 50 years, one to three and more than three positive nodes (P less than 10(-5)). Relapse-free survival also correlated with dose intensity for each of the four prognostic groups analyzed separately (P less than .005). Dose intensity was an independently significant correlate of relapse-free survival in multivariate analysis (P less than 10(-5)). This is a retrospective study, and the hypothesis that dose intensity contributes to outcome independently of other variables should be tested prospectively. Methods of increasing dose intensity also require testing in randomized trials before they can be applied to routine clinical practice.