Specimens of prostate obtained for histopathological and cytological examination by urethral resection and 'punch' biopsies showing carcinoma, atypical and typical hyperplasia were re-examined with regard to the number, size and localization of nucleoli. With increasing degrees of malignancy the number and size of the nucleoli increased and the position of the nucleolus shifted to the periphery of the nucleus. In the cells in typical and slightly atypical prostatic hyperplasia there were few nucleoli which were localized to the centre of the nucleus. In moderately atypical hyperplasia there were intermediate locations, while in severe atypical hyperplasia and 'borderline' malignant cases there was a distinct shift towards the periphery of the nucleus as well as an increase in the number of nucleoli in each nucleus. It is suggested that careful analysis of the number and localization of nucleoli can be helpful in the analysis of the differential diagnosis between carcinoma and atypical hyperplasia of the prostate, which can be difficult in histopathological and cytopathological material.