The validity of diagnostic criteria for the cytologic differentiation between benign and malignant cases and between normal (hyperplastic) and inflammation-activated (prostatitis) prostatic cells was tested by means of semiautomated image analysis and cytophotometry. The following criteria were found to have significant differences between benign and malignant prostatic cells: nucleoli (number, size and size variability), regularity of nuclear arrangement, anisonucleosis, nuclear size, nuclear polymorphism and dissociation of cells. The following criteria were found to be invalid for the detection of prostatic cancer cells: nuclear-cytoplasmic ratio, hyperchromasia and anisochromasia. the cytologic diagnosis of prostatic cancer was achieved by observing a variable constellation of at least three of six pathognomically altered diagnostic criteria. No fixed pattern of diagnostic criteria was found either for prostatic cancer cells in general or for the different malignancy grades.