A prospective randomised clinical trial in patients with node-negative, fast-proliferating breast cancer was initiated in January 1990 to verify the feasibility and reliability of a 3H-thymidine (3H-Tdr) autoradiographic assay in a prospective and consecutive series of node-negative patients and the therapeutic effect of adjuvant chemotherapy in patients with node-negative breast cancer and high tumour proliferative activity. Node-negative patients with a high 3H-Tdr Labelling Index (3H-Tdr-LI) were randomised to receive either no further treatment or combination chemotherapy consisting of fluorouracil, epirubicin plus cyclophosphamide (FEC) for 6 cycles. The autoradiographic assay was performed in 307 of 317 patients (97%) and was evaluable in 291 of 317 patients (92%). A total of 176 patients with a high 3H-Tdr-LI entered the clinical randomised study: 91 in the FEC arm and 85 in the control arm. Patient groups were fairly well balanced regarding the most important clinical and pathological characteristics. In total, 530 FEC cycles have been administered with an actual dose intensity of 90%. Patients receiving FEC demonstrated leucopenia in 35% of cases, alopecia in 70%, and loss of menses in premenopausal patients in an age-dependent manner. Patients are still being entered into the study.