Immunohistochemical staining with the MIB 1 antibody was used to assess cell proliferation in 30 cases of acinic cell carcinoma of salivary glands. Until now, no prognostic factors have been available for these rare tumours. The MIB 1 monoclonal antibody recognizes the Ki-67 antigen in formalin-fixed, paraffin-embedded tissues. A MIB 1 index was developed as a means of expressing the percentage of MIB 1-positive tumour cell nuclei, and the tumours were scored without prior information of clinical behaviour. The staining results were then compared with the clinical outcome of the patients. All eight patients who developed tumour recurrences had MIB 1 indices higher than 5 per cent. Tumour recurrences could be predicted even in cases of bland morphology and low mitotic rate. Three patients died of their recurrent tumours, and had MIB 1 indices of 56.2, 12.7, 7.8 per cent in their primary tumours. Five of seven patients with MIB 1 indices higher than 10 per cent had unfavourable outcomes. None of the 17 patients with MIB 1 indices lower than 5 per cent developed recurrences during follow-up periods up to 30 years. The present results indicate that MIB 1 staining appears to be a significant prognostic factor in acinic cell carcinomas of salivary gland origin.