Aims: The identification of reliable prognostic factors in patients with ovarian stage 1 adult-type granulosa cell tumour (GCT) has proved problematic. Some reports have suggested that proliferation indices may be of value, but the data are conflicting and the methods of assessment often poorly defined. In this study the mitotic activity and Ki-67 immunohistochemistry was assessed in a series of GCT using carefully specified methodology, and the findings were correlated with clinicopathological findings.Methods: Tumour proliferation was assessed in 38 primary GCT by counting mitotic figures in 50 high-power fields (x500 magnification) with results expressed as a mean count per 2 mm(2) standardised area. The number of mitotic figures and Ki-67 immunoreactive cells per 10 000 tumour cells was also assessed using an ocular cell counting graticule. The results were correlated with tumour stage at presentation and with the development of tumour recurrence.Results: Twenty-nine patients were stage 1 at presentation, and nine patients had high-stage disease (extra-ovarian spread). Nine patients with initial stage 1 disease developed metastases, and 20 patients had no evidence of recurrence over a mean follow-up period of 11.1 years. There was no significant correlation between any of the proliferation indices or with clinical outcomes.Conclusions: These results suggest that proliferation assessment is of limited value in the pathological assessment of GCT. Future studies should carefully specify the methods of assessing cell proliferation to ensure a reliable comparison of results.