The influence of a number of risk factors on development of tumour recurrence was studied retrospectively in a group of breast cancer patients without axillary lymph node metastases, all treated identically and with long follow-ups. Of 71 patients all data could be retrieved. This group had a median duration of follow-up of five years. Thirteen patients (18.3%) had had a recurrence of carcinoma after a median disease-free period of 41 months (range 3-124 months). These patients at the time when breast cancer was diagnosed had had a lower median age than patients who had remained free of tumour recurrence, 48 (40-70) as against 59.5 (30-81) years. Factors having a statistically significant prognostic importance for recurrence of tumour were age at the time of diagnosis of breast carcinoma, histological tumour grade and nuclear DNA content. Identifying groups of patients with a higher or lower risk of tumour recurrence appears possible by combination of risk factors. Although patients with breast cancer without axillary lymph node metastases are supposed to have a favourable prognosis, there appear to exist subgroups with a raised risk for development of a recurrence. These groups might be prime candidates for prospective randomized studies of the usefulness of adjuvant therapy.