In the present study, tumor tissue of 251 patients with ovarian cancer was immunohistochemically analysed for proliferative activity using the monoclonal antibody mib-1 and the analyse system CAS 200. The rate of the growth fraction varied from 0% to 71% mib-1 positive tumor cells with a median of 17%. There was a strongly significant association between proliferative activity and the degree of histological differentiation (p = 0.0005), whereas there was no significant correlation with tumor stage and histological subtypes. Using the median as the cut-off point, patients with higher proliferating tumors (> or = 17%) had a statistically significant worse prognosis (p = 0.0431). Especially in the group of patients with grade 1 and grade 2 tumors, measurement of the proliferative activity is of help for prediction of the postoperative survival time of patients (p = 0.0336), suggesting that measurement of the growth fraction estimated by mib-1 reflects more closely the degree of tumor differentiation. However, multivariate analyses cannot confirm these results.