In a multicenter ovarian tumor study, it was shown that there are considerable differences between different pathologists when grading the same ovarian tumors. The question arises whether these differences in grading also reflect prognostic differences. To investigate this, the survival curves of the various tumor grades assigned to the same tumors by four different pathologists were investigated. The results of the present study indicate that tumor grade was strongly correlated with the prognosis, although there were considerable variations in the survival curves and in the five year survival of patients of the same grade assessed by the different pathologists. Five-year survival varied from 82 to 100% in the borderline tumors, from 49 to 80% in the well, and from 21 to 48% in the moderately differentiated tumor groups. Only the poorly differentiated cancers showed less variation. Similar interobserver differences were found in the histological typing and (strictly predefined) malignancy grades. The intraobserver variation, which was also tested after a 6-month interval, was somewhat lower, but was still present for each of the pathologists. These data clearly indicate the necessity for objective, sharply defined, reproducible criteria rather than subjective grades.