The value of the determination of the DNA content of tumor cells for the assessment of the prognosis of mammary adenocarcinoma was studied in 36 patients, who survived for at least 15 years after the cancer had been found and in another 42 patients who died within a 2-year period of diagnosis. The results show a distinct correlation between the type of the DNA histograms of the carcinoma and the grade of cancer. This correlation was particularly apparent among patients surviving for at least 15 years despite the occurrence of metastases or relapses after primary treatment, i.e., in patients not cured by their first treatment.