Intraobserver variation of three grading systems-Mostofi, Gleason and Böcking-is examined. No significant difference was noted between the histological grades found in the two examinations by any of the three methods used. Neither the type of surgical procedure nor the number of slices with tumour influenced the reproducibility of histological grading within each system studied. In the Gleason system the intraobserver highest disagreement would not have resulted in change of therapy choice, but in 2% of tumours graded according to the Mostofi system this would have occurred if the choice of therapy would depend on the grading results. © 1991 Akadémiai Kiadó.235449454