Cell kinetic parameters were determined in 32 patients with breast cancer following intravenous [3H]thymidine injection. The mean labelling index (LI) was 8.6, range 1.2 to 24.1. The LI increased significantly with increased stage of disease. The highest values were noted in patients with chest wall metastases with a mean LI of 15.9. In one patient with multiple discrete tumor foci, the larger foci had significantly decreased LI consistent with classic cytokinetic theory. The intralesional variation in LI was measured in 31 lesions. The average variation of LVlesion was 1.2. Therefore, three samples should provide a representative LI. The interlesional LI was compared by taking multiple samples of two or three lesions per patient in seven patients. For each patient, there was little variation in mean LI from lesion to lesion. The data suggest that higher LI are associated with a poorer prognosis. Among patients with advanced T4 or metastatic disease, the median survivals for patients with LI < 8 and 2 8 were 79 weeks and 21 weeks, respectively. For patients with metastatic disease, the survivals were 56 and 9 weeks, respectively. The tumor growth fraction analyzed in one patient was 40%. In summary, the growth rate of this cancer in an individual patient is relatively homogeneous and LI may provide important prognostic information. Further studies are required to establish the value of LI in the staging and treatment of breast cancer.